.arpa

Infobox Top level domain
name=.arpa
background=#FF9
introduced=1985
type=Infrastructure domain
status=Active
registry=IANA
sponsor=Internet Architecture Board
intendeduse=Address and Routing Parameter Area: Internet infrastructure such as reverse IP lookup.
restrictions=No domain registrations possible, new subdomains rarely added
structure=Second-level domains provide special name spaces for database functions
document=RFC 3172
disputepolicy=None
website= [http://www.iana.org/arpa-dom/ IANA .arpa info] |

.arpa is an Internet top-level domain (TLD) used exclusively for Internet-infrastructure purposes. The name is a backronym for "Address and Routing Parameter Area".

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). 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, although 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 newly introduced, categorized TLDs.

After arpa had served its transitional purpose, it proved impractical to remove the domain, 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 name 155.39.97.145.in-addr.arpa.

At one time, it was intended that new infrastructure databases be created in int. 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").

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

* [http://www.iana.org/arpa-dom/ IANA information on .arpa and its subdomains]
* [http://www.freesoft.org/CIE/RFC/1035/36.htm 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")
* RFC 3405 – Dynamic Delegation Discovery System (DDDS) Part Five: URI.ARPA Assignment Procedures
* [http://www.robert.net/ccTLD/ARPA Zone file analysis]