Arpa

Arpa commonly refers to two related topics in computing and Internet infrastructure: the Advanced Research Projects Agency (ARPA), a United States Department of Defense research agency established in 1958 and renamed DARPA in 1972; and the .arpa top-level domain, a legacy domain used for Internet infrastructure.

ARPA, later known as DARPA, funded research across defense and civilian applications. Its early computer and network initiatives helped develop packet switching, time-sharing, and the ARPANET, the forerunner of the Internet. The agency's projects contributed to TCP/IP adoption in the early 1980s and the broader transition of communications networks to open standards.

The .arpa domain was created in 1985 to support Internet infrastructure management. It is operated under the global DNS coordination framework by IANA in cooperation with ICANN and is not intended for ordinary Web sites. The principal uses are for reverse DNS lookups, including in-addr.arpa for IPv4 and ip6.arpa for IPv6, and for ENUM services under e164.arpa. The domain remains a legacy element of the Internet's technical backbone.

Today, ARPA is known as DARPA and continues to fund high-risk, high-reward research; the .arpa domain remains a restricted infrastructure zone used for essential network operations.