The domain arpa was the first Internet top-level domain. It was intended to be used only temporarily, aiding in the transition of traditional ARPANET host names to the domain name system. However, after it had been used for reverse DNS lookup, it was found impractical to retire it.
The domain name arpa is a top-level domain (TLD) in the Domain Name System of the Internet. It is used exclusively for technical infrastructure purposes. While the name was originally the acronym for the Advanced Research Projects Agency (ARPA), the funding organization in the United States that developed one of the precursors of the Internet (ARPANET), it now stands for Address and Routing Parameter Area.
arpa also contains the domains for reverse domain name resolution in-addr.arpa and ip6.arpa for IPv4 and IPv6, respectively.
As of 2015, IANA distinguishes the following groups of top-level domains:
infrastructure top-level domain (ARPA)
generic top-level domains (gTLD)
restricted generic top-level domains (grTLD)
sponsored top-level domains (sTLD)
country code top-level domains (ccTLD)
test top-level domains (tTLD)
The arpa top-level domain was the first domain installed in the Domain Name System (DNS). It was originally intended to be a temporary domain to facilitate the transition of the ARPANET host naming conventions and the host table distribution methods to the Domain Name System. The ARPANET was one of the predecessors to the Internet, established by the United States Department of Defense Advanced Research Projects Agency (DARPA). When the Domain Name System was introduced in 1985, ARPANET host names were initially converted to domain names by adding the arpa domain name label to the end of the existing host name, separated with a full stop (i.e., a period). Domain names of this form were subsequently rapidly phased out by replacing them with domain names under the newly introduced, categorized top-level domains.
After arpa had served its transitional purpose, it proved impractical to remove the domain, because in-addr.arpa was used for reverse DNS lookup of IP addresses. For example, the mapping of the IP address 126.96.36.199 to a host name is obtained by issuing a DNS query for a pointer record of the domain name 188.8.131.52.in-addr.arpa.
It was intended that new infrastructure databases would be created in the top-level domain int. However, in May 2000 this policy was reversed, and it was decided that arpa should be retained for this purpose, and int should be used solely by international organizations. In accordance with this new policy, arpa now officially stands for Address and Routing Parameter Area (a backronym).
Beginning in March 2010 the arpa zone is signed (DNSSEC).