TLDs are primarily used with the DNS protocol, which maps domain names onto IP addresses. They can be divided into two classes: country code TLDs (ccTLDs) and generic TLDs (gTLDs).
Over two hundred country code TLDs have been established, based on two-letter ISO 3166 country codes (e.g., JP is the TLD for Japan). For further discussion, see the Internet ccTLDs article.
Generic TLDs are (in theory at least) administered globally and available for use by persons from any region. Three of these domains, however, are restricted to users in the United States, for historical reasons.
When top-level domains were first implemented, in January 1985, there were seven gTLDs:
The COM, NET and ORG gTLDs, despite their original different purposes, are now in practice open for use by anybody.
The ARPA TLD was intended to be a temporary measure to facilitate the transition to the Domain Name System. However, removing it completely proved to be impractical, because IN-ADDR.ARPA is the reverse-lookup domain for IPv4 addresses, so it has been retained for Internet-infrastructure purposes. The ARPA TLD no longer has any connection with the ARPA-Internet, and now officially stands for "Address and Routing Parameter Area". Originally, it was intended that new infrastructure databases be created in INT (see below), with a view to eventually deleting ARPA. However, in May 2000 that 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 November 1988, another gTLD was introduced:
The INT gTLD is primarily for international organizations established by international treaties between governments, although it is also used for some Internet infrastructure databases, such as IP6.INT (the IPv6 equivalent of IN-ADDR.ARPA). In May 2000, the [Internet Architecture Board]? proposed that no new infrastructure databases be created in the .INT domain. All future such databases would be created in .ARPA, and existing ones would be moved to .ARPA wherever feasible.
In the year 2001, ICANN began to introduce seven new gTLDs. These are:
As of December 2001, BIZ, INFO, MUSEUM and NAME have been added, although only BIZ and INFO are fully operational.
The ARPANET was a predecessor to the Internet established by the U.S. [Defence Advanced Research Projects Agency]? (DARPA). When the Domain Name System was introduced, ARPANET host names were initially converted to domain names by adding .ARPA to the end. Domain names of this form were rapidly phased out by replacing them with domain names using the other, more informative, TLDs. However, as has been explained above, the ARPA TLD remains in use for other purposes.
There are a few ccTLDs which have been deleted after the corresponding 2-letter code was withdrawn from ISO 3166. Examples include CS (for Czechoslovakia) and ZR (for Zaire). There is usually a significant delay between withdrawal from ISO 3166 and deletion from the DNS. For example, ZR ceased to be an ISO 3166 code in 1997, but the ZR ccTLD was not deleted until 2001, and the SU ccTLD remains in use more than nine years after SU was removed from ISO 3166.
A NATO TLD was added in the late 1980s by the NIC for the use of NATO, who felt that none of the then existing TLDs adequately reflected their status as an international organization. Soon after this addition, however, the NIC created the INT TLD for the use of international organizations, and convinced NATO to use NATO.INT instead. However, the NATO TLD, although no longer used, was not deleted until July 1996.
In the past many computers were not directly connected to the Internet, but could exchange email with it via the Bitnet and UUCP? networks. When used on the Internet, Bitnet and UUCP addresses were given names ending in .bitnet and .uucp, respectively. However, BITNET and UUCP were not real top-level domains and did not exist in DNS.
RFC 2606 reserves four top-level domain names for various purposes, with the intention that these should never become actual TLDs in the global DNS. These are
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