ARPA

ARPA is the acronym for Address and Routing Parameter Area.

Address and Routing Parameter Area

A special infrastructure top-level domain (TLD) in the Domain Name System (DNS). It is not open for public registration and is used to support core Internet services.

Originally, .arpa referred to the Advanced Research Projects Agency, which created ARPANET. It was intended as a temporary domain during the Internet’s transition to DNS in the 1980s, but its critical role—especially in reverse DNS lookups—made it permanent. In 2000, the name was redefined as Address and Routing Parameter Area.

Key uses include:

Reverse DNS lookup: in-addr.arpa for IPv4 and ip6.arpa for IPv6.
Telephone number mapping: e164.arpa for converting E.164 phone numbers to Internet addresses.
Other infrastructure: Domains such as home.arpa, uri.arpa, and resolver.arpa for specific technical functions.

The .arpa domain is managed by the Internet Architecture Board (IAB) and Internet Assigned Numbers Authority (IANA) under guidelines in RFC 3172 and RFC 9120. It remains a cornerstone of Internet stability and routing, quietly powering essential network operations.