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 originally was the acronym for the Advanced Research Projects Agency (ARPA), the funding organization in the United States that developed the precursor of the Internet (ARPANET), it now stands for Address and Routing Parameter Area.
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 the predecessor to the Internet, established by the United States Department of Defense Advanced Research Projects Agency (ARPA). 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. Domain names of this form were subsequently rapidly phased out by replacing them with domain names under the newly introduced, categorized top-level domains.