Wikipedia
.arpa
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.
also contains the domains for reverse domain name resolution and for IPv4 and IPv6, respectively.