.arpa

The .arpa domain is an infrastructure top-level domain used exclusively for technical internet operations. It is sponsored by the Internet Architecture Board (IAB). The name stands for Address and Routing Parameter Area, but the TLD originated as a contraction of ARPANET, the network that preceded the modern internet. .arpa is not a general purpose domain. It is reserved for internet management functions such as reverse DNS lookups (in-addr.arpa and ip6.arpa) and other critical protocols. Only the Internet Assigned Numbers Authority (IANA) can delegate subdomains within .arpa, and there are no public registration services or end user registrations. The TLD was one of the original seven generic top-level domains created in 1985 and was initially intended for ARPANET hosts before being repurposed for infrastructure.

The .arpa domain was the first domain ever created, registered on January 1, 1985. It began as a domain for ARPANET computers. When the ARPANET was decommissioned in the early 1990s, .arpa was not retired. Instead, it was converted into an infrastructure domain used to manage reverse DNS and other technical records. This transition was formalized by the Internet Architecture Board, ensuring that the TLD remains a stable part of the internet's naming system.