Your Internet Consultant - The FAQs of Life Online

6.10. What is a mirror FTP site?

You'll sometimes log on to an FTP site and see a message announcing other sites that are so-called "mirrors" of that system. A mirror site is a computer system that maintains exact duplicates of all the files on some other system. The copy, or mirror, is updated on a regular basis (usually daily or weekly) to ensure that the mirrored information remains up to date.

Mirror sites are useful for a variety of reasons. They reduce the usage load on popular FTP sites by giving users alternative locations to use. Most computers on the Internet can handle only 50 to 100 FTPing users at once, so rather than have 500 people vying for space on a particularly popular site, some of those users can instead connect to mirrors of that site.

Note: You can be reasonably sure that files on a mirror site are the same as the files on the host that is being mirrored, but remember that mirror sites are synchronized on a regular basis--usually daily or weekly--so the very freshest, most recent files may take a day or two to make it to a mirror.

Mirror sites often allow you to connect to a host that's physically closer to your local host, insuring faster, more reliable, less expensive connections. Why should users in the United States connect to a host in Finland (using an expensive trans-Atlantic link) when an exact duplicate of the site is available in their own state? Mirror sites can allow this convenience.

Note: A site may have multiple mirror sites. In fact, the most popular Internet FTP sites have dozens or hundreds of mirrors scattered around the globe.

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