A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet. The top-level domain names are installed in the root zone of the name space. For all domains in lower levels, it is the last part of the domain name, that is, the last label of a fully qualified domain 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. Management of most top-level domains is delegated to responsible organizations by the Internet Corporation for Assigned Names and Numbers (ICANN), which operates the Internet Assigned Numbers Authority (IANA) and is in charge of maintaining the DNS root zone.
Originally, the top-level domain space was organized into three main groups, Countries, Categories, and Multiorganizations. An additional temporary group consisted only of the initial DNS domain, arpa, intended for transitional purposes toward the stabilization of the domain name system.
Countries are designated in the Domain Name System by their two-letter ISO country code; there are exceptions, however (e.g., .uk). This group of domains is therefore commonly known as country-code top-level domains (ccTLD). Since 2009, countries with non-Latin based alphabets or scripting systems may apply for internationalized country code top-level domain names, which are displayed in end-user applications in their language-native script or alphabet, but use a Punycode-translated ASCII domain name in the Domain Name System.
The Categories group has become known as the generic top-level domains. Initially this group consisted of GOV, EDU, COM, MIL, ORG, and NET.
In the growth of the Internet, it became desirable to create additional generic top-level domains. Some of the initial domains' purposes were also generalized, modified, or assigned for maintenance to special organizations affiliated with the intended purpose.
As a result, IANA today distinguishes the following groups of top-level domains:
In addition, a group of internationalized domain name (IDN) top-level domains has been installed under test for testing purposes in the IDN development process.
The authoritative list of currently existing TLDs in the root zone is published at the IANA website at http://www.iana.org/domains/root/db/.
 Internationalized country code TLDs
An internationalized country code top-level domain (IDN ccTLD) is a top-level domain with a specially encoded domain name that is displayed in an end user application, such as a web browser, in its language-native script or alphabet, such as the Arabic alphabet, or a non-alphabetic writing system, such as Chinese characters. IDN ccTLDs are an application of the internationalized domain name (IDN) system to top-level Internet domains assigned to countries, or independent geographic regions.
ICANN started to accept applications for IDN ccTLDs in November 2009, and installed the first set into the Domain Names System in May 2010. The first set was a group of Arabic names for the countries of Egypt, Saudi Arabia, and the United Arab Emirates. By May 2010, 21 countries had submitted applications to ICANN, representing 11 languages.
 Infrastructure domain
The domain arpa was the first Internet top-level domain. It was intended to be used only temporarily, aiding in the transition of traditional ARPANET host names to the domain name system. However, after it had been used for reverse DNS lookup, it was found impractical to retire it, and is used today exclusively for Internet infrastructure purposes such as in-addr.arpa for IPv4 and ip6.arpa for IPv6 reverse DNS resolution, uri.arpa and urn.arpa for the Dynamic Delegation Discovery System, and e164.arpa for telephone number mapping based on NAPTR DNS records. For historical reasons, arpa is sometimes considered to be a generic top-level domain.
 Reserved domains
RFC 2606 reserves the following four top-level domain names to avoid confusion and conflict. They may be used for various specific purposes however, with the intention that these should not occur in production networks within the global domain name system:
- 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 as a hostname
- test: reserved for use in tests
The test domain has seen usage by ICANN in the testing of internationalized domain names, a program started in 2007.
 Historical domains
In the late 1980s InterNIC created the nato domain for use by NATO. NATO considered none of the then existing TLDs as adequately reflecting their status as an international organization. Soon after this addition, however, InterNIC also created the int TLD for the use by international organizations in general, and persuaded NATO to use the second level domain nato.int instead. The nato TLD, no longer used, was finally removed in July 1996.
Other historical TLDs are cs for Czechoslovakia (now cz for Czech Republic and sk for Slovak Republic), dd for East Germany (using de after reunification of Germany), yu for SFR Yugoslavia (now: ba for Bosnia and Herzegovina, hr for Croatia, me for Montenegro, mk for Macedonia, rs for Serbia and si for Slovenia), and zr for Zaire (now cd for Democratic Republic of the Congo). In contrast to these, the TLD su has remained active despite the demise of the Soviet Union that it represents.
 Proposed domains
Around late 2000 when ICANN discussed and finally introduced aero, biz, coop, info, museum, name, and pro TLDs, site owners argued that a similar TLD should be made available for adult and pornographic websites to settle the dispute of obscene content on the Internet and the responsibility of US service providers under the US Communications Decency Act of 1996. Several options were proposed including xxx, sex and adult. As of June 2010, the .xxx TLD has received initial approval from the ICANN, based upon a proposal by the sponsoring agency for this TLD, a Florida-based company called ICM Registry.
An older proposal consisted of seven new gTLDs: arts, firm, info, nom, rec, shop, and web. Later biz, info, museum, and name covered most of these old proposals.
During the 32nd International Public ICANN Meeting in Paris in 2008, ICANN started a new process of TLD naming policy to take a "significant step forward on the introduction of new generic top-level domains." This program envisions the availability of many new or already proposed domains, as well a new application and implementation process. Observers believed that the new rules could result in hundreds of new gTLDs to be registered. Proposed TLDs include music, shop, berlin and nyc.
 Alternative DNS roots
ICANN's slow progress in creating new generic top-level domains, and the high application costs associated with TLDs, contributed to the creation of alternate DNS roots with different sets of top-level domains. Such domains may be accessed by configuration of a computer with alternate or additional (forwarder) DNS servers or plugin modules for web browsers. Browser plugins detect alternate root domain requests and access an alternate domain name server for such requests.
Several networks, such as BITNET, CSNET, UUCP or other networks, existed that were in widespread use among computer professionals and academic users, that were incompatible with the Internet and exchanged e-mail with the Internet via special e-mail gateways. For relaying purposes on the gateways, messages associated with these networks were labeled with suffixes such as bitnet, oz, csnet, or uucp, but these domains did not exist as top-level domains in the public Domain Name System of the Internet.
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. One notable exception is the 2007 emergence of SWIFTNet Mail, which uses the swift pseudo-domain.
The top-level pseudo domain local is required by the Zeroconf protocol. It is also used by many organizations internally, which may 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 emerged.
The anonymity network Tor has a top-level 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 to protect the anonymity of users.
 See also
- ^ RFC 920, Domain Requirements, J. Postel, J. Reynolds, The Internet Society (October 1984)
- ^ RFC 921, Domain Name System Implementation Schedule - Revised, J. Postel, The Internet Society (October 1984)
- ^ Codes for the Representation of Names of Countries, ISO-3166, International Organization for Standardization. (May 1981)
- ^ IANA root zone database
- ^ Internet Corporation For Assigned Names and Numbers (ICANN) (30 October 2009). "ICANN Bringing the Languages of the World to the Global Internet". Press release. http://www.icann.org/en/announcements/announcement-30oct09-en.htm. Retrieved 30 October 2009.
- ^ "'Historic' day as first non-Latin web addresses go live". BBC News. May 6, 2010. http://news.bbc.co.uk/2/hi/technology/10100108.stm. Retrieved 2010-05-07.
- ^ RFC 2606 (BCP 32), Reserved Top Level DNS Names, D. Eastlake, A. Panitz, The Internet Society (June 1999)
- ^ ICANN IDN wiki
- ^ IANA (ICANN) list of extant TLDs
- ^ InterNIC FAQs on New Top-Level Domains
- ^ RFC 3675: .sex Considered Dangerous
- ^ For X-Rated, a Domain of Their Own
- ^ Sex domain .xxx approved by regulators
- ^ (historical) gTLD MoU
- ^ "32nd International Public ICANN Meeting". ICANN. 22 June 2008. http://par.icann.org/.
- ^ "New gTLD Program". ICANN. http://www.icann.org/en/topics/new-gtld-program.htm. Retrieved 15 June 2009.
- ^ ICANN Board Approves Sweeping Overhaul of Top-level Domains, CircleID, 26 June 2008.
- ^ "SWIFTNet Mail now available". SWIFT. 16 May 2007. http://www.swift.com/about_swift/press_room/swift_news_archive/home_page_stories_archive_2007/mail_now_available.page. Retrieved 3 January 2010.
- Addressing the World: National Identity and Internet Country Code Domains, edited by Erica Schlesinger Wass (Rowman & Littlefield, 2003, ISBN 0-7425-2810-3) , examines connections between cultures and their ccTLDs.
- Ruling the Root by Milton Mueller (MIT Press, 2001, ISBN 0-262-13412-8) , discusses TLDs and domain name policy more generally.
- RFC 1591 - Domain Name System Structure and Delegation
- RFC 3071 - Reflections on the DNS, RFC 1591, and Categories of Domains
 External links