The term fedora was in use as early as 1891. Originally a women’s fashion into the 20th century, the fedora came into use in about 1919, as a men’s middle-class clothing accessory. Its popularity soared, and eventually it eclipsed the similar-looking Homburg by the early 1920s. Fedoras can be found in nearly any color imaginable, but black, grey, tan, brown, and red are the most popular.

History

A fedora made by Borsalino

The word fedora comes from the title of an 1882 play by Victorien Sardou, Fédora, written for Sarah Bernhardt. The play was first performed in the U.S. in 1889. Sarah played Princess Fédora, the heroine of the play, and she wore a hat similar to a fedora. The fedora became a female fashion which lasted into the early part of the twentieth century. When the fedora became a male fashion, it was popular in cities for its stylishness, ability to protect the wearer’s head from the wind and weather, and the fact that it could be rolled up when not in use. (Richard Davy, of New York, claimed to be its first male wearer.) Since the early part of the 20th century, many Haredi and other Orthodox Jews have worn black fedoras and continue to this day.

The hat is sometimes associated with Prohibition, Great Depression-era gangsters and the detectives who sought to bring them to justice. Popular stars in the 1950s such as Gene Kelly wore fedoras often in their movies, like Singin’ in the Rain. In Hollywood movies of the 1940s, characters often wore a fedora, particularly when playing private detectives, gangsters, or other “tough guy” roles. A trench coat was frequently part of the costume, a notable example being Humphrey Bogart‘s character in Casablanca. Although the fedora became popular 30 years after the cowboy era (1865–1890), the use of fedoras is common in most TV/movie westerns. The fedora is widely recognized with the characters of The Blues Brothers, Indiana Jones, and Freddy Krueger. The fedora is closely associated with film noir characters.

Like the bowler hat, the fedora was popular from the early 1920s to the mid 1960s on the east coast. In the late 1950s The hat began to lose favor on the west coast of the United States, which is known for its more casual clothing. The late 1950s switch from large lapels and ties to thin ones, resulted in shorter-brimmed hats, and this likely played a role in the fedora eventually being deemed a non-essential item. Also playing a part were the shrinking automobiles of the mid-1950s, which often made it difficult to wear a hat while driving. By the early 1970s, the fedora was seen as a dead fashion, typically only worn by older and/or more traditional men. However the fedora has seen a revival in recent fashion seasons. Instead of the traditional grays, browns, and blacks, the fedora now comes in many colors and patterns, the most popular being plaid, but black with white pinstripes are also common.

Source : en.wikipedia.org

Red Hat has become associated to a large extent with its enterprise operating system Red Hat Enterprise Linux and with the acquisition of open-source enterprise middleware vendor JBoss. Red Hat provides operating-system platforms along with middleware, applications, and management products, as well as support, training, and consulting services.

History

In 1993 Bob Young incorporated the ACC Corporation, a catalog business that sold Linux and UNIX software accessories. In 1994 Marc Ewing created his own Linux distribution, which he named Red Hat Linux. Ewing released it in October, and it became known as the Halloween release. Young bought Ewing’s business in 1995, and the two merged to become Red Hat Software, with Young serving as chief executive officer (CEO).

Red Hat went public on August 11, 1999, the eighth-biggest first-day gain in the history of Wall Street.^[6] Matthew Szulik succeeded Bob Young as CEO in November of that year.

On November 15, 1999, Red Hat acquired Cygnus Solutions. Cygnus provided commercial support for free software and housed maintainers of GNU software products such as the GNU Debugger and GNU Binutils. One of the founders of Cygnus, Michael Tiemann, became the chief technical officer of Red Hat and by 2008^[update] the vice president of open source affairs. Later Red Hat acquired WireSpeed, C2Net and Hell’s Kitchen Systems.^[7]

In February 2000, InfoWorld awarded Red Hat its fourth consecutive^[8] “Operating System Product of the Year” award for Red Hat Linux 6.1. Red Hat acquired Planning Technologies, Inc in 2001 and in 2004 AOL‘s iPlanet directory and certificate-server software.

Red Hat moved its headquarters from Durham, NC, to N.C. State University‘s Centennial Campus in Raleigh, North Carolina in February 2002. In the following month Red Hat introduced the first enterprise-class Linux operating system: Red Hat Linux Advanced Server^[9][10], later re-named Red Hat Enterprise Linux (RHEL). Dell,^[11] IBM,^[12] HP^[13] and Oracle Corporation^[14] announced their support of the platform.^[15]

In December 2005 CIO Insight magazine conducted its annual “Vendor Value Survey”, in which Red Hat ranked #1 in value for the second year in a row.^[16] Red Hat stock became part of the NASDAQ-100 on December 19, 2005.

Red Hat acquired open-source middleware provider JBoss on June 5, 2006 and JBoss became a division of Red Hat. On September 18, 2006, Red Hat released the Red Hat Application Stack, their first stack that integrates the JBoss technology and of which is certified by other well-known software vendors.^[17][18] On December 12, 2006, Red Hat moved from NASDAQ (RHAT) to the New York Stock Exchange (RHT). In 2007 Red Hat acquired MetaMatrix and made an agreement with Exadel to distribute its software.

On March 15, 2007, Red Hat released Red Hat Enterprise Linux 5, and in June acquired Mobicents. On March 13, 2008, Red Hat acquired Amentra, a provider of systems integration services for service-oriented architecture, business process management, systems development and enterprise data services. Amentra operates as an independent Red Hat company.

The Debian Project is governed by the Debian Constitution and the Social Contract which set out the governance structure of the project as well as explicitly stating that the goal of the project is the development of a free operating system. Debian is developed by over one thousand volunteers from around the world and supported by donations through several non-profit organizations around the world. Most important of these is Software in the Public Interest , the owner of the Debian trademark and umbrella organization for various other community free software projects.

Thus, the Debian Project is an independent decentralized organization; it is not backed by a company like other Linux distributions such as Ubuntu, openSUSE, Fedora, and Mandriva. The cost of developing Debian 4.0 etch, assuming paid programmers from a single organization and based on source lines of code, has been estimated to be close to US$13 billion As of April 2, 2009, Ohloh estimates that the Debian GNU/Linux project, assuming a $55,000 average salary, would cost $819,274,547 to redevelop from scratch.

Many distributions are based on Debian, including Ubuntu, MEPIS, Dreamlinux, Damn Small Linux, Xandros, Knoppix, BackTrack, Linspire, sidux, Kanotix, Parsix and LinEx, among others.^[13]

Debian is known for an abundance of options. The current stable release includes over twenty five thousand software packages for twelve^[14] computer architectures. These architectures range from the Intel/AMD 32-bit/64-bit architectures commonly found in personal computers to the ARM architecture commonly found in embedded systems and the IBM eServer zSeries mainframes.^[15] Prominent features of Debian are the APT package management system, repositories with large numbers of packages, strict policies regarding packages, and the high quality of releases.^[13] These practices allow easy upgrades between releases as well as automated installation and removal of packages.

The Debian standard install makes use of the GNOME desktop environment. It includes popular programs such as OpenOffice.org, Iceweasel (a rebranding of Firefox), Evolution mail, CD/DVD writing programs, music and video players, image viewers and editors, and PDF viewers. There are pre-built CD images for KDE, Xfce and LXDE as well.^[16] The remaining discs, which span five DVDs or over thirty CDs, contain all packages currently available and are not necessary for a standard install. Another install method is via a net install CD which is much smaller than a normal install CD/DVD. It contains only the bare essentials needed to start the installer and downloads the packages selected during installation via APT.^[17] These CD/DVD images can be freely obtained by web download, BitTorrent, jigdo or buying them from online retailers.

History

1993–2000

Debian was first announced on 16 August 1993 by Ian Murdock.^[19] Murdock initially called the system “the Debian Linux Release”.^[20] Prior to Debian’s release, the Softlanding Linux System (SLS) had been the first Linux distribution compiled from various software packages, and was a popular basis for other distributions in 1993-1994.^[21] The perceived poor maintenance and prevalence of bugs in SLS^[22] motivated Murdock to launch a new distribution.

In 1993 Murdock also released the Debian Manifesto,^[23] outlining his view for the new operating system. In it he called for the creation of a distribution to be maintained in an open manner, in the spirit of Linux and GNU. He formed the name “Debian” as a combination of the first name of his then girlfriend Debra Lynn and his own first name.^[24] Murdock and Debra later married, then filed for divorce on the week of 2007-08-10.^[25]

The Debian Project grew slowly at first and released the first 0.9x versions in 1994 and 1995. The first ports to other, non-i386 architectures began in 1995, and the first 1.x version of Debian was released in 1996. In 1996, Bruce Perens replaced Ian Murdock as the project leader. In the same year, fellow developer Ean Schuessler suggested that Debian should establish a social contract with its users. He distilled the resulting discussion on Debian mailing lists into the Debian Social Contract and the Debian Free Software Guidelines, defining fundamental commitments for the development of the distribution. He also initiated the creation of the legal umbrella organization, Software in the Public Interest.^[9]

Perens left the project in 1998 before the release of the first glibc-based Debian, 2.0. The Project elected new leaders and made two more 2.x releases, each including more ports and packages. The Advanced Packaging Tool was deployed during this time and the first port to a non-Linux kernel, Debian GNU/Hurd, was started. The first Linux distributions based on Debian, namely Libranet, Corel Linux and Stormix‘s Storm Linux, were started in 1999.^[9]

2000–present

In late 2000, the project made major changes to archive and release management, reorganizing software archive processes with new “package pools” and creating a testing distribution as an ongoing, relatively stable staging area for the next release. In the same year, developers began holding an annual conference called DebConf with talks and workshops for developers and technical users.^[9]

In July 2002, the Project released version 3.0, codenamed woody, a stable release which would see relatively few updates until the following release, 3.1 sarge in June 2005.^[9]

There were many major changes in the sarge release, mostly due to the large time it took to freeze and release the distribution. Not only did this release update over 73% of the software shipped in the previous version, but it also included much more software than previous releases, almost doubling in size with 9,000 new packages. A new installer replaced the aging boot-floppies installer with a modular design. This allowed advanced installations (with RAID, XFS and LVM support) including hardware detection, making installations easier for novice users. The installation system also boasted full internationalization support as the software was translated into almost forty languages. An installation manual and comprehensive release notes were released in ten and fifteen different languages respectively. This release included the efforts of the Debian-Edu/Skolelinux, Debian-Med and Debian-Accessibility sub-projects which boosted the number of educational packages and those with a medical affiliation as well as packages designed especially for people with disabilities.^[9]

Debian 4.0 (etch) was released April 8, 2007 for the same number of architectures as in sarge. It included the AMD64 port but dropped support for m68k. The m68k port was, however, still available in the unstable distribution. There were around 18,200 binary packages maintained by more than 1,030 Debian developers.^[9]

Debian 5.0 (lenny) was released February 14, 2009 after 22 months of development. It includes over 25,000 software packages. Support was added for Marvell’s Orion platform and for netbooks such as the Asus Eee PC.^[1] The release was dedicated to Thiemo Seufer, an active developer and member of the community who died in a car accident on December 26, 2008.^[26]

Mozilla Corporation software rebranded by the Debian project

Firefox and Thunderbird were rebranded in 2006 to Iceweasel and Icedove, along with other Mozilla software. The Mozilla Corporation stated that Debian may not use the Firefox trademark if it distributes Firefox with modifications which have not been approved by the Mozilla Corporation. Two prominent reasons that Debian modifies the Firefox software are to change the artwork, and to provide security patches. Debian’s free software guidelines consider Mozilla’s artwork non-free. Debian provides long term support for older versions of Firefox in the stable release, where Mozilla prefers that old versions are not supported. The software programs owned by the Mozilla Corporation were rebranded but the programs’ source codes remained the same only with minor differences.^[27]

Development procedures

Software packages in development are either uploaded to the project distribution named unstable (also known as sid), or to the experimental repository. Software packages uploaded to unstable are normally versions stable enough to be released by the original upstream developer, but with the added Debian-specific packaging and other modifications introduced by Debian developers. These additions may be new and untested. Software not ready yet for the unstable distribution is typically placed in the experimental repository.^[28]

After a version of a software package has remained in unstable for a certain length of time (depending on the urgency of the software’s changes), that package is automatically migrated to the testing distribution. The package’s migration to testing occurs only if no serious (release-critical) bugs in the package are reported and if other software needed for package functionality qualifies for inclusion in testing.^[28]

Since updates to Debian software packages between official releases do not contain new features, some choose to use the testing and unstable distributions for their newer packages. However, these distributions are less tested than stable, and unstable does not receive timely security updates. In particular, incautious upgrades to working unstable packages can sometimes seriously break software functionality.^[29] Since September 9, 2005^[30] the testing distribution’s security updates have been provided by the testing security team.^[31]

After the packages in testing have matured and the goals for the next release are met, the testing distribution becomes the next stable release. The latest stable release of Debian (lenny) is 5.0, released on February 14, 2009. The forthcoming release is version 6.0, codenamed “Squeeze“.^[28]

Project organization

Diagram of the organizational structure of the project

The Debian Project is a volunteer organization with three foundational documents:

• The Debian Social Contract defines a set of basic principles by which the project and its developers conduct affairs.^[8]
• The Debian Free Software Guidelines define the criteria for “free software” and thus what software is permissible in the distribution, as referenced in the Social Contract. These guidelines have also been adopted as the basis of the Open Source Definition. Although it can be considered a separate document for all practical purposes, it formally is part of the Social Contract.^[8]
• The Debian Constitution describes the organizational structure for formal decision-making within the Project, and enumerates the powers and responsibilities of the Debian Project Leader, the Debian Project Secretary, and the Debian Developers generally.^[7]

Currently, the project includes more than a thousand developers. Each of them sustains some niche in the project, be it package maintenance, software documentation, maintaining the project infrastructure, quality assurance, or release coordination. Package maintainers have jurisdiction over their own packages, although packages are increasingly co-maintained. Other tasks are usually handled by the domain of smaller, more collaborative groups of developers.

The project maintains official mailing lists and conferences for communication and coordination between developers.^[32] For issues with single packages or domains, a public bug tracking system is used by developers and end-users. Informally, Internet Relay Chat channels (primarily on the OFTC and freenode networks) are used for communication among developers and users as well.

Together, the Developers may make binding general decisions by way of a General Resolution or election. All voting is conducted by Cloneproof Schwartz Sequential Dropping, a Condorcet method of voting. A Project Leader is elected once per year by a vote of the Developers; in April 2008, Steve McIntyre was voted into this position, succeeding Sam Hocevar. The Debian Project Leader has several special powers, but this power is far from absolute and is rarely used. Under a General Resolution, the Developers may, among other things, recall the leader, reverse a decision by him or his delegates, and amend the constitution and other foundational documents.

The Leader sometimes delegates authority to other developers in order for them to perform specialized tasks. Generally this means that a leader delegates someone to start a new group for a new task, and gradually a team gets formed that carries on doing the work and regularly expands or reduces their ranks as they think is best and as the circumstances allow.

A role in Debian with a similar importance to the Project Leader’s is that of a Release Manager. Release Managers set goals for the next release, supervise the processes, and make the final decision as to when to release.^{[33] [34]}

Project leaders

The project has had the following leaders:^[35]

1. Ian Murdock (August 1993 – March 1996), founder of the Debian Project
2. Bruce Perens (April 1996 – December 1997)
3. Ian Jackson (January 1998 – December 1998)
4. Wichert Akkerman (January 1999 – March 2001)
5. Ben Collins (April 2001 – April 2002)
6. Bdale Garbee (April 2002 – April 2003)
7. Martin Michlmayr (March 2003 – March 2005)
8. Branden Robinson (April 2005 – April 2006)
9. Anthony Towns (April 2006 – April 2007)
10. Sam Hocevar (April 2007 – April 2008)
11. Steve McIntyre (April 2008 – Present)

A supplemental position, Debian Second in Charge (2IC), was created by Anthony Towns. Steve McIntyre held the position between April 2006 and April 2007. Since April 2009 this position is held by Luk Claes.

Release managers

• Brian C. White (1997–1999)
• Richard Braakman (1999–2000)
• Anthony Towns (2000–2004)
• Steve Langasek, Andreas Barth and Colin Watson (2004–2007)
• Andreas Barth and Luk Claes (2007–2008)
• Luk Claes and Marc Brockschmidt (2008–2009)
• Luk Claes and Adeodato Simó (2009–present)

Note that this list includes the active release managers; it does not include the release assistants (first introduced in 2003) and the retiring managers (“release wizards”).^[33]

Developer recruitment, motivation, and resignation

The Debian project has a steady influx of applicants wishing to become developers. These applicants must undergo an elaborate vetting process which establishes their identity, motivation, understanding of the project’s goals (embodied in the Social Contract), and technical competence.^[36]

Debian Developers join the Project for a number of reasons; some that have been cited in the past include:^[37]

• A desire to contribute back to the Free Software community (practically all applicants are users of Free Software)
• A desire to see some specific software task accomplished (some view the Debian user community as a valuable testing or proving ground for new software)
• A desire to make, or keep, Free Software competitive with proprietary alternatives
• A desire to work closely with people that share some of their aptitudes, interests, and goals (there is a very strong sense of community within the Debian project which some applicants do not experience in their paid jobs)
• A simple enjoyment of the iterative process of software development and maintenance

Debian Developers may resign their positions at any time by orphaning the packages they were responsible for and sending a notice to the developers and the keyring maintainer (so that their upload authorization can be revoked).

Package life cycle

Flowchart of the life cycle of a Debian package

Each Debian software package has a maintainer who keeps track of releases by the “upstream” authors of the software and ensures that the package is compliant with Debian Policy, coheres with the rest of the distribution, and meets the standards of quality of Debian. In relations with users and other developers, the maintainer uses the bug tracking system to follow up on bug reports and fix bugs. Typically, there is only one maintainer for a single package, but increasingly small teams of developers “co-maintain” larger and more complex packages and groups of packages.^[38]

Periodically, a package maintainer makes a release of a package by uploading it to the “incoming” directory of the Debian package archive (or an “upload queue” which periodically batch-transmits packages to the incoming directory). Package uploads are automatically processed to ensure that they are well-formed (all the requisite files are in place) and that the package is digitally signed by a Debian developer using OpenPGP-compatible software. All Debian developers have public keys.^[39] Packages are signed to be able to reject uploads from hostile outsiders to the project, and to permit accountability in the event that a package contains a serious bug, a violation of policy, or malicious code.

If the package in incoming is found to be validly signed and well-formed, it is installed into the archive into an area called the “pool” and distributed every day to hundreds of mirrors worldwide. Initially, all package uploads accepted into the archive are only available in the “unstable” suite of packages, which contains the most up-to-date version of each package.

However, new code is also untried code, and those packages are only distributed with clear disclaimers. For packages to become candidates for the next “stable” release of the Debian distribution, they first need to be included in the “testing” suite. The requirements for a package to be included in “testing” is that it:^{[40] [41]}

• Must have been in unstable for the appropriate length of time (the exact duration depends on the “urgency” of the upload)
• Must not have a greater number of “release-critical” bugs filed against it than the current version in testing. Release-critical bugs are those bugs which are considered serious enough that they make the package unsuitable for release.
• Must be compiled for all release architectures the package claims to support (eg: the i386-specific package gmod can be included in “testing”)
• All of its dependencies must either be satisfiable by packages already in testing, or be satisfiable by the group of packages which are going to be installed at the same time.
• The operation of installing the package into testing must not break any packages currently in testing.

Thus, a release-critical bug in a package on which many packages depend, such as a shared library, may prevent many packages from entering the “testing” area, because that library is considered deficient.

Periodically, the Release Manager publishes guidelines to the developers in order to ready the release, and in accordance with them eventually decides to make a release. This occurs when all important software is reasonably up-to-date in the release-candidate suite for all architectures for which a release is planned, and when any other goals set by the Release Manager have been met. At that time, all packages in the release-candidate suite (“testing”) become part of the released suite (“stable”).

It is possible for a package – particularly an old, stable, and seldom-updated one – to belong to more than one suite at the same time. The suites are simply collections of pointers into the package “pool” mentioned above.

Security information and policy

The Debian Project, being free software, handles security policy through public disclosure rather than through security through obscurity. Many advisories are coordinated with other free software vendors and are published the same day a vulnerability is made public. Debian has a security audit team that reviews the archive looking for new or unfixed security bugs. Debian also participates in security standardization efforts: the Debian security advisories are compatible with the Common Vulnerabilities and Exposures (CVE) dictionary, and Debian is represented in the Board of the Open Vulnerability and Assessment Language (OVAL) project.^[42]

The Debian Project offers extensive documentation and tools to harden a Debian installation both manually and automatically.^[43] SELinux (Security-Enhanced Linux) packages are installed by default though not enabled.

Source : en.wikipedia.org

History

[edit] 1993–2000

Debian was first announced on 16 August 1993 by Ian Murdock.^[19] Murdock initially called the system “the Debian Linux Release”.^[20] Prior to Debian’s release, the Softlanding Linux System (SLS) had been the first Linux distribution compiled from various software packages, and was a popular basis for other distributions in 1993-1994.^[21] The perceived poor maintenance and prevalence of bugs in SLS^[22] motivated Murdock to launch a new distribution.

In 1993 Murdock also released the Debian Manifesto,^[23] outlining his view for the new operating system. In it he called for the creation of a distribution to be maintained in an open manner, in the spirit of Linux and GNU. He formed the name “Debian” as a combination of the first name of his then girlfriend Debra Lynn and his own first name.^[24] Murdock and Debra later married, then filed for divorce on the week of 2007-08-10.^[25]

The Debian Project grew slowly at first and released the first 0.9x versions in 1994 and 1995. The first ports to other, non-i386 architectures began in 1995, and the first 1.x version of Debian was released in 1996. In 1996, Bruce Perens replaced Ian Murdock as the project leader. In the same year, fellow developer Ean Schuessler suggested that Debian should establish a social contract with its users. He distilled the resulting discussion on Debian mailing lists into the Debian Social Contract and the Debian Free Software Guidelines, defining fundamental commitments for the development of the distribution. He also initiated the creation of the legal umbrella organization, Software in the Public Interest.^[9]

Perens left the project in 1998 before the release of the first glibc-based Debian, 2.0. The Project elected new leaders and made two more 2.x releases, each including more ports and packages. The Advanced Packaging Tool was deployed during this time and the first port to a non-Linux kernel, Debian GNU/Hurd, was started. The first Linux distributions based on Debian, namely Libranet, Corel Linux and Stormix‘s Storm Linux, were started in 1999.^[9]

[edit] 2000–present

In late 2000, the project made major changes to archive and release management, reorganizing software archive processes with new “package pools” and creating a testing distribution as an ongoing, relatively stable staging area for the next release. In the same year, developers began holding an annual conference called DebConf with talks and workshops for developers and technical users.^[9]

In July 2002, the Project released version 3.0, codenamed woody, a stable release which would see relatively few updates until the following release, 3.1 sarge in June 2005.^[9]

There were many major changes in the sarge release, mostly due to the large time it took to freeze and release the distribution. Not only did this release update over 73% of the software shipped in the previous version, but it also included much more software than previous releases, almost doubling in size with 9,000 new packages. A new installer replaced the aging boot-floppies installer with a modular design. This allowed advanced installations (with RAID, XFS and LVM support) including hardware detection, making installations easier for novice users. The installation system also boasted full internationalization support as the software was translated into almost forty languages. An installation manual and comprehensive release notes were released in ten and fifteen different languages respectively. This release included the efforts of the Debian-Edu/Skolelinux, Debian-Med and Debian-Accessibility sub-projects which boosted the number of educational packages and those with a medical affiliation as well as packages designed especially for people with disabilities.^[9]

Debian 4.0 (etch) was released April 8, 2007 for the same number of architectures as in sarge. It included the AMD64 port but dropped support for m68k. The m68k port was, however, still available in the unstable distribution. There were around 18,200 binary packages maintained by more than 1,030 Debian developers.^[9]

Debian 5.0 (lenny) was released February 14, 2009 after 22 months of development. It includes over 25,000 software packages. Support was added for Marvell’s Orion platform and for netbooks such as the Asus Eee PC.^[1] The release was dedicated to Thiemo Seufer, an active developer and member of the community who died in a car accident on December 26, 2008.^[26]

[edit] Mozilla Corporation software rebranded by the Debian project

Main article: Mozilla Corporation software rebranded by the Debian project

Firefox and Thunderbird were rebranded in 2006 to Iceweasel and Icedove, along with other Mozilla software. The Mozilla Corporation stated that Debian may not use the Firefox trademark if it distributes Firefox with modifications which have not been approved by the Mozilla Corporation. Two prominent reasons that Debian modifies the Firefox software are to change the artwork, and to provide security patches. Debian’s free software guidelines consider Mozilla’s artwork non-free. Debian provides long term support for older versions of Firefox in the stable release, where Mozilla prefers that old versions are not supported. The software programs owned by the Mozilla Corporation were rebranded but the programs’ source codes remained the same only with minor differences.^[27]

[edit] Development procedures

Software packages in development are either uploaded to the project distribution named unstable (also known as sid), or to the experimental repository. Software packages uploaded to unstable are normally versions stable enough to be released by the original upstream developer, but with the added Debian-specific packaging and other modifications introduced by Debian developers. These additions may be new and untested. Software not ready yet for the unstable distribution is typically placed in the experimental repository.^[28]

After a version of a software package has remained in unstable for a certain length of time (depending on the urgency of the software’s changes), that package is automatically migrated to the testing distribution. The package’s migration to testing occurs only if no serious (release-critical) bugs in the package are reported and if other software needed for package functionality qualifies for inclusion in testing.^[28]

Since updates to Debian software packages between official releases do not contain new features, some choose to use the testing and unstable distributions for their newer packages. However, these distributions are less tested than stable, and unstable does not receive timely security updates. In particular, incautious upgrades to working unstable packages can sometimes seriously break software functionality.^[29] Since September 9, 2005^[30] the testing distribution’s security updates have been provided by the testing security team.^[31]

After the packages in testing have matured and the goals for the next release are met, the testing distribution becomes the next stable release. The latest stable release of Debian (lenny) is 5.0, released on February 14, 2009. The forthcoming release is version 6.0, codenamed “Squeeze“.^[28]

[edit] Project organization

Diagram of the organizational structure of the project

The Debian Project is a volunteer organization with three foundational documents:

• The Debian Social Contract defines a set of basic principles by which the project and its developers conduct affairs.^[8]
• The Debian Free Software Guidelines define the criteria for “free software” and thus what software is permissible in the distribution, as referenced in the Social Contract. These guidelines have also been adopted as the basis of the Open Source Definition. Although it can be considered a separate document for all practical purposes, it formally is part of the Social Contract.^[8]
• The Debian Constitution describes the organizational structure for formal decision-making within the Project, and enumerates the powers and responsibilities of the Debian Project Leader, the Debian Project Secretary, and the Debian Developers generally.^[7]

Currently, the project includes more than a thousand developers. Each of them sustains some niche in the project, be it package maintenance, software documentation, maintaining the project infrastructure, quality assurance, or release coordination. Package maintainers have jurisdiction over their own packages, although packages are increasingly co-maintained. Other tasks are usually handled by the domain of smaller, more collaborative groups of developers.

The project maintains official mailing lists and conferences for communication and coordination between developers.^[32] For issues with single packages or domains, a public bug tracking system is used by developers and end-users. Informally, Internet Relay Chat channels (primarily on the OFTC and freenode networks) are used for communication among developers and users as well.

Together, the Developers may make binding general decisions by way of a General Resolution or election. All voting is conducted by Cloneproof Schwartz Sequential Dropping, a Condorcet method of voting. A Project Leader is elected once per year by a vote of the Developers; in April 2008, Steve McIntyre was voted into this position, succeeding Sam Hocevar. The Debian Project Leader has several special powers, but this power is far from absolute and is rarely used. Under a General Resolution, the Developers may, among other things, recall the leader, reverse a decision by him or his delegates, and amend the constitution and other foundational documents.

The Leader sometimes delegates authority to other developers in order for them to perform specialized tasks. Generally this means that a leader delegates someone to start a new group for a new task, and gradually a team gets formed that carries on doing the work and regularly expands or reduces their ranks as they think is best and as the circumstances allow.

A role in Debian with a similar importance to the Project Leader’s is that of a Release Manager. Release Managers set goals for the next release, supervise the processes, and make the final decision as to when to release.^{[33] [34]}

[edit] Project leaders

The project has had the following leaders:^[35]

1. Ian Murdock (August 1993 – March 1996), founder of the Debian Project
2. Bruce Perens (April 1996 – December 1997)
3. Ian Jackson (January 1998 – December 1998)
4. Wichert Akkerman (January 1999 – March 2001)
5. Ben Collins (April 2001 – April 2002)
6. Bdale Garbee (April 2002 – April 2003)
7. Martin Michlmayr (March 2003 – March 2005)
8. Branden Robinson (April 2005 – April 2006)
9. Anthony Towns (April 2006 – April 2007)
10. Sam Hocevar (April 2007 – April 2008)
11. Steve McIntyre (April 2008 – Present)

A supplemental position, Debian Second in Charge (2IC), was created by Anthony Towns. Steve McIntyre held the position between April 2006 and April 2007. Since April 2009 this position is held by Luk Claes.

[edit] Release managers

• Brian C. White (1997–1999)
• Richard Braakman (1999–2000)
• Anthony Towns (2000–2004)
• Steve Langasek, Andreas Barth and Colin Watson (2004–2007)
• Andreas Barth and Luk Claes (2007–2008)
• Luk Claes and Marc Brockschmidt (2008–2009)
• Luk Claes and Adeodato Simó (2009–present)

Note that this list includes the active release managers; it does not include the release assistants (first introduced in 2003) and the retiring managers (“release wizards”).^[33]

[edit] Developer recruitment, motivation, and resignation

The Debian project has a steady influx of applicants wishing to become developers. These applicants must undergo an elaborate vetting process which establishes their identity, motivation, understanding of the project’s goals (embodied in the Social Contract), and technical competence.^[36]

Debian Developers join the Project for a number of reasons; some that have been cited in the past include:^[37]

• A desire to contribute back to the Free Software community (practically all applicants are users of Free Software)
• A desire to see some specific software task accomplished (some view the Debian user community as a valuable testing or proving ground for new software)
• A desire to make, or keep, Free Software competitive with proprietary alternatives
• A desire to work closely with people that share some of their aptitudes, interests, and goals (there is a very strong sense of community within the Debian project which some applicants do not experience in their paid jobs)
• A simple enjoyment of the iterative process of software development and maintenance

Debian Developers may resign their positions at any time by orphaning the packages they were responsible for and sending a notice to the developers and the keyring maintainer (so that their upload authorization can be revoked).

[edit] Package life cycle

Flowchart of the life cycle of a Debian package

Each Debian software package has a maintainer who keeps track of releases by the “upstream” authors of the software and ensures that the package is compliant with Debian Policy, coheres with the rest of the distribution, and meets the standards of quality of Debian. In relations with users and other developers, the maintainer uses the bug tracking system to follow up on bug reports and fix bugs. Typically, there is only one maintainer for a single package, but increasingly small teams of developers “co-maintain” larger and more complex packages and groups of packages.^[38]

Periodically, a package maintainer makes a release of a package by uploading it to the “incoming” directory of the Debian package archive (or an “upload queue” which periodically batch-transmits packages to the incoming directory). Package uploads are automatically processed to ensure that they are well-formed (all the requisite files are in place) and that the package is digitally signed by a Debian developer using OpenPGP-compatible software. All Debian developers have public keys.^[39] Packages are signed to be able to reject uploads from hostile outsiders to the project, and to permit accountability in the event that a package contains a serious bug, a violation of policy, or malicious code.

If the package in incoming is found to be validly signed and well-formed, it is installed into the archive into an area called the “pool” and distributed every day to hundreds of mirrors worldwide. Initially, all package uploads accepted into the archive are only available in the “unstable” suite of packages, which contains the most up-to-date version of each package.

However, new code is also untried code, and those packages are only distributed with clear disclaimers. For packages to become candidates for the next “stable” release of the Debian distribution, they first need to be included in the “testing” suite. The requirements for a package to be included in “testing” is that it:^{[40] [41]}

• Must have been in unstable for the appropriate length of time (the exact duration depends on the “urgency” of the upload)
• Must not have a greater number of “release-critical” bugs filed against it than the current version in testing. Release-critical bugs are those bugs which are considered serious enough that they make the package unsuitable for release.
• Must be compiled for all release architectures the package claims to support (eg: the i386-specific package gmod can be included in “testing”)
• All of its dependencies must either be satisfiable by packages already in testing, or be satisfiable by the group of packages which are going to be installed at the same time.
• The operation of installing the package into testing must not break any packages currently in testing.

Thus, a release-critical bug in a package on which many packages depend, such as a shared library, may prevent many packages from entering the “testing” area, because that library is considered deficient.

Periodically, the Release Manager publishes guidelines to the developers in order to ready the release, and in accordance with them eventually decides to make a release. This occurs when all important software is reasonably up-to-date in the release-candidate suite for all architectures for which a release is planned, and when any other goals set by the Release Manager have been met. At that time, all packages in the release-candidate suite (“testing”) become part of the released suite (“stable”).

It is possible for a package – particularly an old, stable, and seldom-updated one – to belong to more than one suite at the same time. The suites are simply collections of pointers into the package “pool” mentioned above.

[edit] Security information and policy

The Debian Project, being free software, handles security policy through public disclosure rather than through security through obscurity. Many advisories are coordinated with other free software vendors and are published the same day a vulnerability is made public. Debian has a security audit team that reviews the archive looking for new or unfixed security bugs. Debian also participates in security standardization efforts: the Debian security advisories are compatible with the Common Vulnerabilities and Exposures (CVE) dictionary, and Debian is represented in the Board of the Open Vulnerability and Assessment Language (OVAL) project.^[42]

The Debian Project offers extensive documentation and tools to harden a Debian installation both manually and automatically.^[43] SELinux (Security-Enhanced Linux) packages are installed by default though not enabled.

Slackware aims for design stability and simplicity, and to be the most “Unix-like” Linux distribution, using plain text files for configuration and making as few modifications to software packages as possible from upstream.

Name

The name “Slackware” stems from the fact that the distribution started as a private side project with no intended commitment. To prevent it from being taken too seriously at first, Volkerding gave it a humorous name, which stuck even after Slackware became a serious project.^[3]

The “slack” in Slackware is a reference to the term “slack” as used by the Church of the SubGenius.

History

Slackware was originally descended from the Softlanding Linux System, the most popular of the original Linux distributions. SLS dominated the market until the developers made a decision to change the executable format from a.out to ELF. This was not a popular decision among SLS’s user base at the time. Patrick Volkerding released a modified version of SLS, which he named Slackware.^[5] The first Slackware release, 1.00, was on July 16, 1993.^[6] It was supplied as 3½” floppy disk images that were available by anonymous FTP.

In 1999, Slackware’s release numbers saw a large increment from 4 to 7. This was explained by Patrick Volkerding as a marketing effort to show that Slackware was as up-to-date as other Linux distributions, many of which had release numbers of 6 at the time (such as Red Hat releasing each revision of its distribution with an increment of 4.1 to 5.0 instead of 3.1 to 3.2 as Slackware did).^[7]

In 2005, the GNOME desktop environment was removed from the pending future release, and turned over to community support and distribution.^[8] The removal of GNOME was seen by some in the Linux community as significant because the desktop environment is found in many Linux distributions. In lieu of this, several community-based projects began offering complete GNOME distributions for Slackware.

Package management

Slackware mascot

Slackware’s package management system can install, upgrade, and remove packages from local sources, but makes no attempt to track or manage dependencies, relying on the user to ensure that the system has all the supporting system libraries and programs required by the new package. If any of these are missing, there may be no indication until one attempts to use the newly installed software.

Slackware packages are gzipped tarballs with filenames ending with .tgz. The package contains the files that form part of the software being installed, as well as additional files for the benefit of the Slackware package manager. The files that form part of the software being installed are organized such that, when extracted into the root directory, their files are placed in their installed locations. The other files are those placed under the install/ directory inside the package.

Two files are commonly found in the install/ directory, which are the slack-desc and doinst.sh files. These are not placed directly into the filesystem in the same manner as the other files in the package. The slack-desc file is a simple text file which contains a description of the package being installed. This is used when viewing packages using the package manager. The doinst.sh file is a shell script which is usually intended to run commands or make changes which could not be best made by changing the contents of the package. This script is run at the end of the installation of a package.

Source : en.wikipedia.org

The network allows computers to communicate with each other and share resources and information. The Advanced Research Projects Agency (ARPA) designed “Advanced Research Projects Agency Network” (ARPANET) for the United States Department of Defense. It was the first computer network in the world in late 1960s and early 1970s.^[1]

Network classification

The following list presents categories used for classifying networks.

Connection method

Computer networks can also be classified according to the hardware and software technology that is used to interconnect the individual devices in the network, such as Optical fiber, Ethernet, Wireless LAN, HomePNA, Power line communication or G.hn. Ethernet uses physical wiring to connect devices. Frequently deployed devices include hubs, switches, bridges and/or routers.

Wireless LAN technology is designed to connect devices without wiring. These devices use radio waves or infrared signals as a transmission medium.

ITU-T G.hn technology uses existing home wiring (coaxial cable, phone lines and power lines) to create a high-speed (up to 1 Gigabit/s) local area network.
Wired Technologies

Twisted-Pair Wire – This is the most widely used medium for telecommunication. Twisted-pair wires are ordinary telephone wires which consist of two insulated copper wires twisted into pairs and are used for both voice and data transmission. The use of two wires twisted together helps to reduce crosstalk and electromagnetic induction. The transmission speed range from 2 million bits per second to 100 million bits per second.
Coaxial Cable – These cables are widely used for cable television systems, office buildings, and other worksites for local area networks. The cables consist of copper or aluminum wire wrapped with insulating layer typically of a flexible material with a high dielectric constant, all of which are surrounded by a conductive layer. The layers of insulation help minimize interference and distortion. Transmission speed range from 200 million to more than 500 million bits per second.
Fiber Optics – These cables consist of one or more thin filaments of glass fiber wrapped in a protective layer. It transmits light which can travel over long distance and higher bandwidths. Fiber-optic cables are not affected by electromagnetic radiation. Transmission speed could go up to as high as trillions of bits per second. The speed of fiber optics is hundreds of times faster than coaxial cables and thousands of times faster than twisted-pair wire.

Wireless Technologies

Terrestrial Microwave – Terrestrial microwaves use Earth-based transmitter and receiver. The equipments look like satellite dish. Terrestrial microwaves use low-gigahertz range, which limits all communications to line-of-sight. Path between relay stations spaced approx. 30 miles apart. Microwave antennas are usually placed on top of buildings, towers, hills, and mountain peaks.
Communications Satellites – The satellites use microwave radio as their telecommunications medium which are not deflected by the Earth’s atmosphere. The satellites are stationed in space, typically 22,000 miles above the equator. These Earth-orbiting systems are capable of receiving and relaying voice, data, and TV signals.
Cellular and PCS Systems – Use several radio communications technologies. The systems are divided to different geographic area. Each area has low-power transmitter or radio relay antenna device to relay calls from one area to the next area.
Wireless LANs – Wireless local area network use a high-frequency radio technology similar to digital cellular and a low-frequency radio technology. Wireless LANS use spread spectrum technology to enable communication between multiple devices in a limited area. Example of open-standard wireless radio-wave technology is IEEE 802.11b.
Bluetooth – A short range wireless technology. Operate at approx. 1Mbps with range from 10 to 100 meters. Bluetooth is an open wireless protocol for data exchange over short distances.
The Wireless Web – The wireless web refers to the use of the World Wide Web through equipments like cellular phones, pagers, PDAs, and other portable communications devices. The wireless web service offers anytime/anywhere connection.

Scale

Networks are often classified as Local Area Network (LAN), Wide Area Network (WAN), Metropolitan Area Network (MAN), Personal Area Network (PAN), Virtual Private Network (VPN), Campus Area Network (CAN), Storage Area Network (SAN), etc. depending on their scale, scope and purpose. Usage, trust levels and access rights often differ between these types of network – for example, LANs tend to be designed for internal use by an organization’s internal systems and employees in individual physical locations (such as a building), while WANs may connect physically separate parts of an organization to each other and may include connections to third parties.

Functional relationship (network architecture)

Computer networks may be classified according to the functional relationships which exist among the elements of the network, e.g., Active Networking, Client-server and Peer-to-peer (workgroup) architecture.

Network topology

Computer networks may be classified according to the network topology upon which the network is based, such as bus network, star network, ring network, mesh network, star-bus network, tree or hierarchical topology network. Network topology signifies the way in which devices in the network see their logical relations to one another. The use of the term “logical” here is significant. That is, network topology is independent of the “physical” layout of the network. Even if networked computers are physically placed in a linear arrangement, if they are connected via a hub, the network has a Star topology, rather than a bus topology. In this regard the visual and operational characteristics of a network are distinct; the logical network topology is not necessarily the same as the physical layout. Networks may be classified based on the method of data used to convey the data, these include digital and analog networks.

Types of networks

Below is a list of the most common types of computer networks in order of scale.

Personal area network

A personal area network (PAN) is a computer network used for communication among computer devices close to one person. Some examples of devices that are used in a PAN are printers, fax machines, telephones, PDAs and scanners. The reach of a PAN is typically about 20-30 feet (approximately 6-9 meters), but this is expected to increase with technology improvements.

Local area network

A local area network (LAN) is a computer network covering a small physical area, like a home, office, or small group of buildings, such as a school, or an airport. Current wired LANs are most likely to be based on Ethernet technology, although new standards like ITU-T G.hn also provide a way to create a wired LAN using existing home wires (coaxial cables, phone lines and power lines)^[2].

For example, a library may have a wired or wireless LAN for users to interconnect local devices (e.g., printers and servers) and to connect to the internet. On a wired LAN, PCs in the library are typically connected by category 5 (Cat5) cable, running the IEEE 802.3 protocol through a system of interconnected devices and eventually connect to the Internet. The cables to the servers are typically on Cat 5e enhanced cable, which will support IEEE 802.3 at 1 Gbit/s. A wireless LAN may exist using a different IEEE protocol, 802.11b, 802.11g or possibly 802.11n. The staff computers (bright green in the figure) can get to the color printer, checkout records, and the academic network and the Internet. All user computers can get to the Internet and the card catalog. Each workgroup can get to its local printer. Note that the printers are not accessible from outside their workgroup.

Typical library network, in a branching tree topology and controlled access to resources

All interconnected devices must understand the network layer (layer 3), because they are handling multiple subnets (the different colors). Those inside the library, which have only 10/100 Mbit/s Ethernet connections to the user device and a Gigabit Ethernet connection to the central router, could be called “layer 3 switches” because they only have Ethernet interfaces and must understand IP. It would be more correct to call them access routers, where the router at the top is a distribution router that connects to the Internet and academic networks’ customer access routers.
The defining characteristics of LANs, in contrast to WANs (wide area networks), include their higher data transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines. Current Ethernet or other IEEE 802.3 LAN technologies operate at speeds up to 10 Gbit/s. This is the data transfer rate. IEEE has projects investigating the standardization of 40 and 100 Gbit/s.^[3]

Campus area network

A campus area network (CAN) is a computer network made up of an interconnection of local area networks (LANs) within a limited geographical area. It can be considered one form of a metropolitan area network, specific to an academic setting.

In the case of a university campus-based campus area network, the network is likely to link a variety of campus buildings including; academic departments, the university library and student residence halls. A campus area network is larger than a local area network but smaller than a wide area network (WAN) (in some cases).

The main aim of a campus area network is to facilitate students accessing internet and university resources. This is a network that connects two or more LANs but that is limited to a specific and contiguous geographical area such as a college campus, industrial complex, office building, or a military base. A CAN may be considered a type of MAN (metropolitan area network), but is generally limited to a smaller area than a typical MAN. This term is most often used to discuss the implementation of networks for a contiguous area. This should not be confused with a Controller Area Network. A LAN connects network devices over a relatively short distance. A networked office building, school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs (perhaps one per room), and occasionally a LAN will span a group of nearby buildings.

Metropolitan area network

A metropolitan area network (MAN) is a network that connects two or more local area networks or campus area networks together but does not extend beyond the boundaries of the immediate town/city. Routers, switches and hubs are connected to create a metropolitan area network.

Wide area network

A wide area network (WAN) is a computer network that covers a broad area (i.e. any network whose communications links cross metropolitan, regional, or national boundaries [1]). Less formally, a WAN is a network that uses routers and public communications links Contrast with personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs), which are usually limited to a room, building, campus or specific metropolitan area (e.g., a city) respectively. The largest and most well-known example of a WAN is the Internet. A WAN is a data communications network that covers a relatively broad geographic area (i.e. one city to another and one country to another country) and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.

Global area network

A global area networks (GAN) (see also IEEE 802.20) specification is in development by several groups, and there is no common definition. In general, however, a GAN is a model for supporting mobile communications across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is “handing off” the user communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial WIRELESS local area networks (WLAN).^[4]

Virtual private network

A virtual private network (VPN) is a computer network in which some of the links between nodes are carried by open connections or virtual circuits in some larger network (e.g., the Internet) instead of by physical wires. The data link layer protocols of the virtual network are said to be tunneled through the larger network when this is the case. One common application is secure communications through the public Internet, but a VPN need not have explicit security features, such as authentication or content encryption. VPNs, for example, can be used to separate the traffic of different user communities over an underlying network with strong security features.

A VPN may have best-effort performance, or may have a defined service level agreement (SLA) between the VPN customer and the VPN service provider. Generally, a VPN has a topology more complex than point-to-point.

A VPN allows computer users to appear to be editing from an IP address location other than the one which connects the actual computer to the Internet.

Internetwork

An Internetwork is the connection of two or more distinct computer networks or network segments via a common routing technology. The result is called an internetwork (often shortened to internet). Two or more networks or network segments connected using devices that operate at layer 3 (the ‘network’ layer) of the OSI Basic Reference Model, such as a router. Any interconnection among or between public, private, commercial, industrial, or governmental networks may also be defined as an internetwork.

In modern practice, interconnected networks use the Internet Protocol. There are at least three variants of internetworks, depending on who administers and who participates in them:

• Intranet
• Extranet
• Internet

Intranets and extranets may or may not have connections to the Internet. If connected to the Internet, the intranet or extranet is normally protected from being accessed from the Internet without proper authorization. The Internet is not considered to be a part of the intranet or extranet, although it may serve as a portal for access to portions of an extranet.

Intranet

An intranet is a set of networks, using the Internet Protocol and IP-based tools such as web browsers and file transfer applications, that is under the control of a single administrative entity. That administrative entity closes the intranet to all but specific, authorized users. Most commonly, an intranet is the internal network of an organization. A large intranet will typically have at least one web server to provide users with organizational information.

Extranet

An extranet is a network or internetwork that is limited in scope to a single organization or entity but which also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities (e.g., a company’s customers may be given access to some part of its intranet creating in this way an extranet, while at the same time the customers may not be considered ‘trusted’ from a security standpoint). Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other type of network, although, by definition, an extranet cannot consist of a single LAN; it must have at least one connection with an external network.

Internet

The Internet consists of a worldwide interconnection of governmental, academic, public, and private networks based upon the networking technologies of the Internet Protocol Suite. It is the successor of the Advanced Research Projects Agency Network (ARPANET) developed by DARPA of the U.S. Department of Defense. The Internet is also the communications backbone underlying the World Wide Web (WWW). The ‘Internet’ is most commonly spelled with a capital ‘I’ as a proper noun, for historical reasons and to distinguish it from other generic internetworks.

Participants in the Internet use a diverse array of methods of several hundred documented, and often standardized, protocols compatible with the Internet Protocol Suite and an addressing system (IP Addresses) administered by the Internet Assigned Numbers Authority and address registries. Service providers and large enterprises exchange information about the reachability of their address spaces through the Border Gateway Protocol (BGP), forming a redundant worldwide mesh of transmission paths.

Basic hardware components

All networks are made up of basic hardware building blocks to interconnect network nodes, such as Network Interface Cards (NICs), Bridges, Hubs, Switches, and Routers. In addition, some method of connecting these building blocks is required, usually in the form of galvanic cable (most commonly Category 5 cable). Less common are microwave links (as in IEEE 802.12) or optical cable (“optical fiber“). An ethernet card may also be required.

Network interface cards

A network card, network adapter, or NIC (network interface card) is a piece of computer hardware designed to allow computers to communicate over a computer network. It provides physical access to a networking medium and often provides a low-level addressing system through the use of MAC addresses.

Repeaters

A repeater is an electronic device that receives a signal and retransmits it at a higher power level, or to the other side of an obstruction, so that the signal can cover longer distances without degradation. In most twisted pair Ethernet configurations, repeaters are required for cable which runs longer than 100 meters.

Hubs

A network hub contains multiple ports. When a packet arrives at one port, it is copied unmodified to all ports of the hub for transmission. The destination address in the frame is not changed to a broadcast address.^[5]

Bridges

A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. Bridges do not promiscuously copy traffic to all ports, as hubs do, but learn which MAC addresses are reachable through specific ports. Once the bridge associates a port and an address, it will send traffic for that address only to that port. Bridges do send broadcasts to all ports except the one on which the broadcast was received.

Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports. Once a frame arrives through a port, its source address is stored and the bridge assumes that MAC address is associated with that port. The first time that a previously unknown destination address is seen, the bridge will forward the frame to all ports other than the one on which the frame arrived.

Bridges come in three basic types:

1. Local bridges: Directly connect local area networks (LANs)
2. Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote bridges, where the connecting link is slower than the end networks, largely have been replaced by routers.
3. Wireless bridges: Can be used to join LANs or connect remote stations to LANs.

Switches

A network switch is a device that forwards and filters OSI layer 2 datagrams (chunk of data communication) between ports (connected cables) based on the MAC addresses in the packets.^[6] This is distinct from a hub in that it only forwards the packets to the ports involved in the communications rather than all ports connected. Strictly speaking, a switch is not capable of routing traffic based on IP address (OSI Layer 3) which is necessary for communicating between network segments or within a large or complex LAN. Some switches are capable of routing based on IP addresses but are still called switches as a marketing term. A switch normally has numerous ports, with the intention being that most or all of the network is connected directly to the switch, or another switch that is in turn connected to a switch.^[7]

Switch is a marketing term that encompasses routers and bridges, as well as devices that may distribute traffic on load or by application content (e.g., a Web URL identifier). Switches may operate at one or more OSI model layers, including physical, data link, network, or transport (i.e., end-to-end). A device that operates simultaneously at more than one of these layers is called a multilayer switch.

Overemphasizing the ill-defined term “switch” often leads to confusion when first trying to understand networking. Many experienced network designers and operators recommend starting with the logic of devices dealing with only one protocol level, not all of which are covered by OSI. Multilayer device selection is an advanced topic that may lead to selecting particular implementations, but multilayer switching is simply not a real-world design concept.

Routers

A router is a networking device that forwards packets between networks using information in protocol headers and forwarding tables to determine the best next router for each packet. Routers work at the Network Layer of the OSI model and the Internet Layer of TCP/IP.

Source : en.wikipedia.org