Overview#RFC 2606 is a RFC describing TLDs for Testing, & Documentation Examples
Ldapwiki refers you to the full text.
There is a need for Top-Level Domain (TLD) names that can be used for creating names which, without fear of conflicts with current or future actual Top-Level Domain names in the global DNS, can be used for private testing of existing DNS related code, examples in documentation, DNS related experimentation, invalid DNS names, or other similar uses.
For example, without guidance, a site might set up some local additional unused top level domains for testing of its local DNS code and configuration. Later, these TLDs might come into actual use on the global Internet. As a result, local attempts to reference the real data in these zones could be thwarted by the local test versions. Or test or example code might be written that accesses a TLD that is in use with the thought that the test code would only be run in a restricted testbed net or the example never actually run.
Later, the test code could escape from the testbed or the example be actually coded and run on the Internet. Depending on the nature of the test or example, it might be best for it to be referencing a TLD permanently reserved for such purposes.
To safely satisfy these needs, four domain names are reserved as listed and described below.
.test .example .invalid .localhost
".example" is recommended for use in documentation or as examples.
".invalid" is intended for use in online construction of domain names that are sure to be invalid and which it is obvious at a glance are invalid.
The ".localhost" TLD has traditionally been statically defined in host DNS implementations as having an A record pointing to the loop back IP address and is reserved for such use. Any other use would conflict with widely deployed code which assumes this use.
Reserved Example Second Level Domain Names#The Internet Assigned Numbers Authority (IANA) also currently has the following second level domain names reserved which can be used as examples.
example.com example.net example.org