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'.arpa' is an Internet top-level domain (TLD) used exclusively for Internet-infrastructure purposes. It does not function as a normal TLD where websites are registered, but rather as a meta-TLD used to look up addresses, and for other purposes.
The .arpa TLD was originally intended to be a temporary measure to facilitate the transition 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), and when the Domain Name System was introduced in 1985, ARPANET host names were initially converted to domain names by adding .arpa to the end. Hostnames in other networks were also sometimes converted to pseudo-domain-style addresses by adding endings such as .uucp and .bitnet, though these were never added to the Internet root as formal TLDs. Domain names of this form were rapidly phased out by replacing them with domain names using the other, more informative, TLDs.
However, deleting .arpa once it had served its transitional purpose proved to be impractical, because in-addr.arpa was used for reverse DNS lookup for IP addresses. For example the IP address 145.97.39.155 is mapped to a host name by issuing a DNS query for the PTR record for the special host name 155.39.97.145.in-addr.arpa.
At one time, it was intended that new infrastructure databases be created in .int, with a view to eventually deleting .arpa. However, in May 2000 this policy was reversed, and it was decided that .arpa should be retained for this purpose, and .int should be retained solely for the use of international organizations. In accordance with this new policy, .arpa now officially stands for 'Address and Routing Parameter Area' (a "backronym").

Contents

Second-level domains

External links

Second-level domains

★ e164.arpa - ENUM lookup mapping telephone numbers into DNS

★ in-addr.arpa - reverse DNS lookup for IPv4 addresses

★ iris.arpa - For use in CRISP

★ ip6.arpa - reverse DNS lookup for IPv6 addresses

★ uri.arpa - for dynamic discovery of URI addressing schemes

★ urn.arpa - for dynamic discovery of URN addressing schemes

External links

★ IANA information on .arpa and its subdomains

★ Description of in-addr.arpa domain in RFC 1035 (DNS specification)

★ RFC 3172 – Management Guidelines & Operational Requirements for the Address and Routing Parameter Area Domain ("arpa")