ARPA

.arpa (address and routing parameter area) is a top-level domain in the Domain Name System (DNS) used for Internet infrastructure purposes, especially reverse DNS lookup (i.e., find the domain name for a given IP address).

The name came from ARPA, currently known as DARPA, the Defense Advanced Research Projects Agency. DARPA is credited with developing precursors to the Internet (ARPANET), GPS, artificial intelligence, and virtual reality. The domain was originally intended only to serve a temporary function for facilitating the systematic naming of the ARPANET computers, but failed to remove it after infrastructural uses had been sanctioned. As a result, the ARPA domain name is a backronym for "Address and Routing Parameter Area".

TLD was designed at DARPA during a time when it was called ARPA, hence the .arpa domain name.

The most prominent .arpa domains include in-addr.arpa and ip6.arpa, which provide namespaces for reverse DNS lookup of IPv4 and IPv6 addresses respectively.