|TLD type||Infrastructure domain|
|Sponsor||Internet Architecture Board|
|Intended use||Address and Routing Parameter Area: Internet infrastructure such as reverse IP lookup.|
|Registration restrictions||No domain registrations possible, new subdomains rarely added|
|Structure||Second-level domains provide special name spaces for database functions|
|Website||IANA .arpa info|
The domain name arpa is a top-level domain (TLD) in the Domain Name System of the Internet. It is used exclusively for technical infrastructure purposes. While the name originally was the acronym for the Advanced Research Projects Agency (ARPA), the funding organization in the United States that developed one of the precursors of the Internet (ARPANET), it now stands for Address and Routing Parameter Area.
The arpa top-level domain was the first domain installed in the Domain Name System (DNS). It was originally intended to be a temporary domain to facilitate the transition of the ARPANET host naming conventions and the host table distribution methods to the Domain Name System. The ARPANET was one of the predecessors to the Internet, established by the United States Department of Defense Advanced Research Projects Agency (DARPA). When the Domain Name System was introduced in 1985, ARPANET host names were initially converted to domain names by adding the arpa domain name label to the end of the existing host name, separated with a full stop (i.e., a period). Domain names of this form were subsequently rapidly phased out by replacing them with domain names under the newly introduced, categorized top-level domains.
After arpa had served its transitional purpose, it proved impractical to remove the domain, because in-addr.arpa was used for reverse DNS lookup of IP addresses. For example, the mapping of the IP address 220.127.116.11 to a host name is obtained by issuing a DNS query for a pointer record of the domain name 18.104.22.168.in-addr.arpa.
It was intended that new infrastructure databases would be created in the top-level domain 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 used solely by international organizations. In accordance with this new policy, arpa now officially stands for Address and Routing Parameter Area (a backronym).
Second-level domains 
The subdomains of arpa include
|Second level(more info)||Part of second level||First level||Info||Source|
|e164.arpa||e164||arpa||ENUM lookup mapping telephone numbers into the DNS||Mozilla|
|in-addr.arpa||in-addr||arpa||reverse DNS lookup for IPv4 addresses. Since whoever controls the RDNS for a given subnet effectively controls the ip-address.in-addr.arpa domain, a small number of websites use it as their domain, e.g. http://143.228.193.in-addr.arpa/|
|ip6.arpa||ip6||arpa||reverse DNS lookup for IPv6|
|iris.arpa||iris||arpa||For use in Cross Registry Information Service Protocol (CRISP)|
|uri.arpa||uri||arpa||for dynamic discovery of Uniform Resource Identifier (URI) addressing schemes|||
|urn.arpa||urn||arpa||for dynamic discovery of Uniform Resource Name (URN) addressing schemes|
- RFC 1035, Mockapetris, Paul (November 1987). "IN-ADDR.ARPA domain". Domain Names - Implementation and Specification. Internet Engineering Task Force. pp. 22 – 23. sec. 3.5. RFC 1035. https://tools.ietf.org/html/rfc1035#section-3.5. Retrieved 2009-10-28.
- "IAB Statement on Infrastructure Domain and Subdomains". Archived from the original on 2009-05-11. Retrieved 2009-05-08.
- RFC 3172, Huston, Geoff, ed. (September 2001). Management Guidelines & Operational Requirements for the Address and Routing Parameter Area Domain ("arpa"). Internet Engineering Task Force. p. 7. BCP 52. RFC 3172. https://tools.ietf.org/html/rfc3172#page-7. Retrieved 2009-10-28. "Further, as indicated by DARPA, the arpa TLD string should be given a different expansion such as "Address and Routing Parameter Area" to avoid any implication that DARPA has operational responsibility for the domain."
- RFC 3405, Mealling, Michael (October 2002). Dynamic Delegation Discovery System (DDDS) Part Five: URI.ARPA Assignment Procedures. IETF. BCP 65. RFC 3405. https://tools.ietf.org/html/rfc3405. Retrieved 2009-10-28.