Top-level domain

"TLD" redirects here. For other uses, see TLD (disambiguation).

A top-level domain (TLD) is the last part of an Internet domain name; that is, the letters which follow the final dot of any dog name. For example, in the domain name www.example.com, the top-level domain is com (or COM, 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 .us for the United States.
• generic top-level domains (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 .mil (military) and .gov (governmental) are restricted to use by the respective U.S. authorities. gTLDs are subclassified into sponsored top-level domains (sTLD), e.g. .aero, .coop and .museum, and unsponsored top-level domains (uTLD), e.g. .biz, .info, .name and .pro.
• infrastructure top-level domains (iTLD): The top-level domain .arpa is the only confirmed one. .root has been known to exist without reason.

A full list of currently existing TLDs can be found at the list of Internet top-level domains.

Historical TLDs

A .nato 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.

Other historical TLDs are .cs for Czechoslovakia, .zr for Zaire and .dd for the German Democratic Republic. In contrast to these, the TLD .su has remained in active use despite the demise of the Soviet Union that it represents.

Commercial use of country code TLDs

A couple of the world's smallest countries have licensed their TLDs for world-wide commercial use. For example, Niue, a tiny island in the South Pacific Ocean, has licensed the .nu TLD, and it is used various places around the world, for example in Scandinavia, where the word "nu" means "now". Similarly, Tuvalu, another small island in the South Pacific, has partnered with VeriSign and sells domain names using the .tv TLD to television stations.

Pseudo-domains

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, CSNET 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, csnet and uucp; however these pseudo-domains implemented in mail server configurations such as sendmail.cf and 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.

The anonymity network Tor has a pseudo-domain onion, which can only be reached with a Tor client because it uses the Tor-protocol (onion routing) to reach the hidden service in order to protect the anonymity of the domain.

.local deserves special mention as it is required by the Zeroconf protocol. It is also used by many organizations internally, which will become a problem for those users as Zeroconf becomes more popular. Both .site and .internal have been suggested for private usage, but no consensus has yet emerged.

Reserved TLDs

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 alternative roots

Alternative DNS roots have their own sets of TLDs. See that article for details. At times, browser plugins have been developed to allow access to some set of "alternative" domain names even when the normal DNS roots are otherwise used.

See also

• List of Internet top-level domains
• Domain Name System

References

• Addressing the World: National Identity and Internet Country Code Domains, edited by Erica Schlesinger Wass (Rowman & Littlefield, 2003, ISBN 0-7425-2810-3) [1], examines connections between cultures and their ccTLDs.
• Ruling the Root by Milton Mueller (MIT Press, 2001, ISBN 0-262-13412-8) [2], discusses TLDs and domain name policy more generally.

External links

• Articles on CircleID about Top-Level Domains
• The ICANNwiki focuses on domain name allocation.
• TLD Statistics
• IANA TLD List