Every domain name on the Internet consists of some names separated by dots, and the last name is called the top level domain, or TLD. For example, in the domain name www.example.com the TLD is com (or COM, as TLDs are not case-sensitive).
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). ccTLDs are always two letters long, gTLDs are always longer than two letters.
See also: List of Internet TLDs.
A country code top-level domain (or ccTLD) is a top-level domain established for the use of a country or dependent area (e.g., JP for Japan). The rules regarding who is entitled to domains in the ccTLD are developed by ccTLD managers, who are also responsible for the operation of the domain. In some cases anyone in the world can acquire a domain in the ccTLD; in other cases, only residents of the country or dependent area are allowed to have a domain in it. The ccTLD manager is appointed by IANA, with the permission of the government of the country or dependent area.
Over 240 of these ccTLDs have been established (http://www.iana.org/cctld/cctld-whois.htm). They are based on two-letter ISO 3166-1 country codes, although there are some differences; three codes exist in ISO 3166-1 but are not used as ccTLDs, while others not in ISO 3166-1 are nonetheless used as ccTLDs.
The ISO 3166-1 codes EH and KP, although theoretically available as ccTLDs for Western Sahara and North Korea, have never been assigned and do not exist in DNS. Also, the new ISO 3166-1 code TL (for East Timor) is not yet used as a ccTLD. On the other hand, seven ccTLDs are currently in use despite not being ISO 3166-1 two-letter codes, namely AC (Ascension Island), GG (Guernsey), IM (Isle of Man), JE (Jersey), SU (Soviet Union), TP (East Timor) and UK (United Kingdom). TP is the previous ISO 3166-1 code for East Timor. SU is an obsolete ISO 3166-1 code which has remained in use as a ccTLD alongside the new ccTLDs for the former Soviet republics. The use of the UK ccTLD dates back to the early days of the Internet before the policy of using ISO 3166-1 codes had been settled. The use of the codes AC, GG, IM and JE as ccTLDs arose from IANA's decision in 1996 to allow the use of codes reserved in the ISO 3166-1 alpha-2 reserve list for use by the Universal Postal Union. The decision was later reversed, and only these four ccTLDs were assigned under this rule.
On 25 September 2000, ICANN decided to allow the use of any two-letter code in the ISO 3166-1 reserve list that is reserved for all purposes. Only EU (for the European Union) currently meets this criterion. Following a decision by the EU's Council of Telecommunications Ministers in March 2002, it seems likely that an EU ccTLD will be established in due course, perhaps in 2003.
Some ccTLDs (GB and SU), exist in the root zone, but are not on IANA's list of ccTLDs  (http://www.iana.org/cctld/cctld-whois.htm). GB is used by a very few sites, and no new registrations are being accepted for it. SU is one of the more heavily used TLDs. The SU ccTLD (USSR) manager has recently (2001) stated they will commence accepting new SU registrations, but it is unclear whether this action is compatible with ICANN policy.
Generic top-level domains 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.
By the mid-1990s there was pressure for more gTLDs to be introduced. The first attempt at this involved the setting up of a temporary organization called the International Ad Hoc Committee (IAHC). On 4 February 1997, the IAHC issued a report recommending the introduction of seven new gTLDs (ARTS, FIRM, INFO, NOM, REC, STORE and WEB). However, progress on this became stalled for political and business reasons and nothing ever became of it. In October 1998, the Internet Corporation for Assigned Names and Numbers (ICANN) was formed to take over the task of managing domain names. After a call for proposals (15 August 2000) and a brief period of public consultation, ICANN announced on 16 November 2000 its selection of seven new gTLDs. These were:
These new gTLDs began to be introduced in June 2001, and by the end of that year all except PRO had been added, with BIZ, INFO and MUSEUM already in full operation. NAME and COOP became fully operational in January 2002, and AERO is scheduled to follow later in the year. PRO was added in May 2002.
Besides ICANN, several other organizations operate root servers, each with its own set of TLDs. Among them are:
The BIZ TLD run by Pacific Root was in operation before ICANN proposed running BIZ, so some of the alternate root servers resolve BIZ to PacRoot's. There are .BIZ domain names that exist in both roots and point to different IP addresses.
The ARPANET was a predecessor to the Internet established by the U.S. Defense 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 including reverse DNS lookup where the IP address ww.xx.yy.zz maps to zz.yy.xx.ww.in-addr.arpa.
There are a few ccTLDs which have been deleted after the corresponding 2-letter code was withdrawn from ISO_3166-1. Examples include CS (for Czechoslovakia) and ZR (for Zaire). There is usually a significant delay between withdrawal from ISO 3166-1 and deletion from the DNS. For example, ZR ceased to be an ISO 3166-1 code in 1997, but the ZR ccTLD was not deleted until 2001, and the SU (Soviet Union) ccTLD remains in use more than nine years after SU was removed from ISO 3166-1.
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 the Internet was just one of many wide area computer networks. Computers not connected to the Internet, but connected to another network such as Bitnet or UUCP could generally exchange e-mail with the Internet via e-mail gateways. When used on the Internet, addresses on these networks were often placed under pseudo-domains such as .bitnet and .uucp; however these pseudo-domains were not real top-level domains and did not exist in DNS.
Most of these networks have long since ceased to exist, and although UUCP is still in significant use in parts of the world where Internet infrastructure is not well-established, it subsequently transitioned to using Internet domain names, so pseudo-domains are are now largely historical.
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