Mirror sites or mirrors are replicas of other websites or any network node. The concept of mirroring applies to network services accessible through any protocol, such as HTTP or FTP. Such sites have different URLs than the original site, but host identical or near-identical content. Mirror sites are often located in a different geographic region than the original, or upstream site. The purpose of mirrors is to reduce network traffic, improve access speed, ensure availability of the original site for technical or political reasons, or provide a real-time backup of the original site. Mirror sites are particularly important in developing countries, where internet access may be slower or less reliable. The maintainers of some mirrors choose not to replicate the entire contents of the upstream server they are mirroring because of technical constraints, or selecting only a subset relevant to their purpose, such as software written in a particular programming language, runnable on a single computer platform, or written by one author. These sites are called partial mirrors or secondary mirrors.
Mirror sites were heavily used on the early internet, when most users accessed through dialup and the Internet backbone had much lower bandwidth than today, making a geographically-localized mirror network a worthwhile benefit. Download archives such as Info-Mac, Tucows and CPAN maintained worldwide networks mirroring their content accessible over HTTP or anonymous FTP. Some of these networks, such as Info-Mac or Tucows are no longer active or have removed their mirrored download sections, but some like CPAN or the Debian package mirrors are still active in 2019. Debian removed FTP access to its mirrors in 2017 because of declining use and the relative stagnation of the FTP protocol, mentioning FTP servers' lack of support for techniques such as caching and load balancing that are available to HTTP.
Notable websites with mirrors include Project Gutenberg, KickassTorrents, The Pirate Bay, WikiLeaks, the website of the Environmental Protection Agency, and Wikipedia. Some notable partial mirrors include free and open-source software projects such as GNU, in particular Linux distributions such as Debian and Fedora; such projects provide mirrors of the download sites (since those are expected to have high load), but not do necessarily mirror the main websites.
It was once common for tech companies such as Microsoft, Hewlet-Packard or Apple Computer to maintain a network of mirrors accessible over HTTP or anonymous FTP, hosting software updates, sample code and various freely-downloadable utilities. Much of these sites were shut down in the first decades of the 21st century, with Apple shutting down its FTP services in 2012 and Microsoft stopping updates in 2010. Today, the contents of a number of these mirror sites are archived at https://archive.org/details/ftpsites&tab=collection
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- "Debian worldwide mirror sites".
Using a nearby server will probably speed up your download, and also reduce the load on our central servers and on the Internet as a whole.
- "Impending Trump has Internet Archive mirror to Canada". 2016-11-29.
The Internet Archive has several mirrors up right now, and Canada is set to be its next. This move is taking place specifically because of the new presidential elect Trump here in the United States.
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We all become frustrated when web pages take minutes to unfold. This can increase the gap between infrastructure haves and have-nots. Downloading time is important for other reasons; users connecting to the internet via telephone line in many countries are charged per minute and slow downloading itself may make users lose interest.
- "Debian worldwide mirror sites".
A secondary mirror site may have restrictions on what they mirror
- "Debian Project to Shut Down Its Public FTP Services, Developers Are Not Affected".
The decision to close the Debian FTP services for users was made because the FTP servers in their current state lack support for acceleration or caching, and they aren't quite used lately due to the fact that the Debian Installer no longer provides an FTP option for accessing mirrors since more than ten years ago... FTP as a protocol appears to no longer be efficient, requiring adding strange workarounds to firewalls and load-balancing daemons.
- "Project Gutenberg, nonprofit organization".
In addition, dozens of “mirror” Web sites were created around the world, where the e-books were also stored and available for downloading.
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- "How do I access the MICROSOFT FTP server?".