.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 comes 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.
TLD was designed at DARPA during a time when it was called ARPA, hence the
.arpa domain name.
The most prominent
.arpa domains include
ip6.arpa, which provide namespaces for reverse DNS lookup of IPv4 and IPv6 addresses respectively.