A top-level domain (TLD) is the last part of which Internet domain names consist of. For example, in the domain name wikipedia.org the top-level domain is org (or ORG, as domain names are not case-sensitive).
The Internet Assigned Numbers Authority (IANA) currently classifies top-level domains into three types:
country code top-level domains (ccTLD): used by a country or a dependent territory. It is two letters long, for example jp for Japan.
generic top-level domain (gTLD): used (at least in theory) by a particular class of organizations (for example, com for commercial organizations). It is three or more letters long. Most gTLDs are available for use worldwide, but for historical reasons gov and mil are restricted to the government and military of the USA respectively.
- infrastructure top-level domain: the top-level domain arpa is the only one
A full list of currently existing TLDs can be found at the list of Internet top-level domains.
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 still gets significant use in parts of the world where Internet infrastructure has not yet become well-established, it subsequently transitioned to using Internet domain names, so pseudo-domains now largely survive as historical relics.
RFC 2606 reserves the following four top-level domain names for various purposes, with the intention that these should never become actual TLDs in the global DNS:
- example — reserved for use in examples
- invalid — reserved for use in obviously invalid domain names
- localhost — reserved to avoid conflict with the traditional use of localhost
- test — reserved for use in tests
TLDs in alternate roots
Alternate DNS roots have their own sets of TLDs. See that article for details.
Addressing the World: National Identity and Internet Country Code Domains, edited by Erica Schlesinger Wass (Rowman & Littlefield, 2003, ISBN 0742528103) , examines connections between cultures and their ccTLDs.
Ruling the Root by Milton Mueller (MIT Press, 2001, ISBN 0262134128) , discusses TLDs and domain name policy more generally.