.arpa is an Internet top-level domain (TLD) used exclusively for Internet-infrastructure purposes.
The .arpa TLD was originally intended to be a temporary measure to facilitate the transition to the Domain Name System. The ARPANET was the predecessor to the Internet established by the United States Department of Defense Advanced Research Projects Agency (ARPA), and when the Domain Name System was introduced in 1985, ARPANET host names were initially converted to domain names by adding .arpa to the end. Domain names of this form were rapidly phased out by replacing them with domain names using the other, more informative, TLDs.
However, deleting .arpa once it had served its transitional purpose proved to be impractical, because in-addr.arpa was used for reverse DNS lookup for IP addresses. For example the IP address 212.30.222.56 is mapped to a host name by issuing a DNS query for the PTR record for the special host name 56.222.30.212.in-addr.arpa.
At one time, it was intended that new infrastructure databases be created in .int, with a view to eventually deleting .arpa. However, in May 2000 this policy was reversed, and it was decided that .arpa should be retained for this purpose, and .int should be retained solely for the use of international organizations. In accordance with this new policy, .arpa now officially stands for Address and Routing Parameter Area.
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