The Domain Name System (DNS) is a crucial naming system for computers/resources/services on the Internet. The DNS has a hierarchical structure, and the highest level within it is the root zone; the root zone is managed by ICANN and IANA (IANA is a department within ICANN. The root zone contains a root zone file that has information about top-level domains (TLDs); the most well known TLD is com (dotcom). A less well known, but technically more important top-level domain, is the .arpa top-level domain. According to RFC 3172, the arpa domain was created when the DNS was first implemented (1983-1984), and its purpose was to transition the Internet from the host table of the ARPANET network to the Domain Name System (DNS). Therefore, originally, the arpa domain was largely a technical mechanism that involved IPv4 reverse mapping.
As the Internet evolved and grew -- such as adding IPv6 -- and the infrastructure of ARPANET was discarded, the role of the arpa domain was redesignated, to become a "Address and Routing Parameter Area" (RFC 3172, Year: 2000). The arpa domain is currently managed by two Internet organisations: 1) Internet Architecture Board (IAB); 2) Internet Corporation for Assigned Names and Numbers (ICANN). RFC 3172 states that it is the Internet Architecture Board (IAB) that decides which protocol elements are placed within the arpa domain, these are then managed by a "appropriate protocol management entity"; presumable meaning IANA. The Internet Assigned Numbers Authority (IANA) is an important department within ICANN -- it predates ICANN and was once responsible for managing large parts of the DNS -- and is currently the department within ICANN that administers the arpa domain/registry. IANA currently refer to the arpa domain as a: Address and Routing Parameter Area that is mostly used for Internet infrastructure purposes.
The arpa domain currently contains the following second-level domains: as112.arpa, e164.arpa, home.arpa, in-addr-servers.arpa, in-addr.arpa, ip6-servers.arpa, ip6.arpa, ipv4only.arpa, iris.arpa, uri.arpa, and urn.arpa. The second-level domains within the arpa top-level domain fulfill a range of technical duties, such as: reverse IP address lookups; to map E.164 numbers to URIs; to host in-addr.arpa authoritative name servers; to resolve Uniform Resource Names; to locate Internet Registry Information Services; to detect DNS64; for protocol translation of the IPv6 prefix; to map IPv6 addresses to domain names; to map IPv4 addresses to domain names; and to host ip6.arpa authoritative name servers.
It should not be understated how important the arpa domain is to the functioning of the Internet, it creates an Internet that is stable, integrated and efficient. It is sometimes questioned why the arpa domain is a top-level domain: RFC 3172 explains that the arpa domain is a top-level domain to avoid the instability that could be caused by multiple operational domain lookups. The servers that host the arpa domain are viewed as critical, and therefore have the same operational requirements as root zone servers. In 2001, many authoritative servers for the root zone were also serving as authoritative servers for the arpa domain/zone.