Mirror sites or mirrors are replicas of other websites. The concept of mirroring applies to network services accessible through any protocol, such as HTTP and FTP
These websites 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 on the main site, improve access speed, ensure availability of the original site for technical 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 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Some of these networks, such as Info-Mac or Tucows are no longer active or have removed their mirrored download sections, but some like CPAN or the Debian package mirrors are still active as of 2019. Debian removed FTP access to its mirrors in 2017 because of declining use and the relative stagnation of the FTP protocol.
Anyway, as you can see the concept has many benifits to a website, even today.
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