files/en-us/glossary/arpa/index.md
.arpa (address and routing parameter area) is a {{glossary("TLD","top-level domain")}} in the Domain Name System (DNS) used for Internet infrastructure purposes, especially reverse DNS lookup (i.e., find the {{glossary('domain name')}} for a given {{glossary("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 ({{glossary("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".
{{glossary("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.