A top-level domain (TLD), sometimes referred to as a top-level domain name (TLDN), is the last part of an Internet domain name; that is, the letters that follow the final dot of any 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 parties or 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.

IANA currently distinguishes the following groups of top-level domains:

• infrastructure top-level domain: This group consists of one TLD, the Address and Routing Parameter Area (ARPA). It is managed by IANA on behalf of the IETF for various purposes specified in RFCs.
• 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. With some historical exceptions, the code for any territory is the same as its two-letter ISO 3166 code.
• sponsored top-level domains (sTLD): These domains are proposed and sponsored by private agencies or organizations that establish and enforce rules restricting the eligibility to use the TLD. Often they are also grouped with the generic top-level domains.
• generic top-level domains (gTLD): Generic domains are essentially open for registration to anyone in the world.
• generic-restricted top-level domains: Similar to generic group, except eligibility is supposed to be restricted and ascertained more stringently.

In addition, a group of internationalized domain name (IDN) top-level domains has been installed for testing purposes.

The authoritative list of currently existing TLDs in the root zone can be found at the IANA website at http://www.iana.org/domains/root/db/ and a Wikipedia list exists.

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 persuaded 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 (now .cz for Czech Republic and .sk for Slovak Republic), 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, though .ru is most commonly used for Russian domains.

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

In 2007 eleven IDN test TLDs were created:

1. .xn--kgbechtv       Arabic (.إختبار)
2. .xn--hgbk6aj7f53bba Persian (.آزمایشی)
3. .xn--0zwm56d        Chinese, simplified (.测试)
4. .xn--g6w251d        Chinese, traditional (.測試)
5. .xn--80akhbyknj4f   Cyrillic (.испытание)
6. .xn--11b5bs3a9aj6g  Hindi (.परीक्षा)
7. .xn--jxalpdlp       Greek (.δοκιμή)
8. .xn--9t4b11yi5a     Korean (.테스트)
9. .xn--deba0ad        Yiddish (.טעסט)
10. .xn--zckzah         Japanese (.テスト)
11. .xn--hlcj6aya9esc7a Tamil (.பரிட்சை)

Infrastructure TLD

Today .arpa is used exclusively for Internet-infrastructure purposes such as in-addr.arpa for IPv4 and ip6.arpa for IPv6 reverse DNS lookup, uri.arpa and urn.arpa for the Dynamic Delegation Discovery System, and e164.arpa for Telephone Number Mapping based on NAPTR records. For historical reasons, .arpa is sometimes considered to be a generic TLD.

Debated TLDs

About the time that ICANN discussed and finally introduced .aero, .biz, .coop, .info, .museum, .name, and .pro TLDs, site owners and USENET users 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 service providers under the questionable Communications Decency Act of 1996. Several options were proposed including .xxx, .sex and .adult, but so far ICANN has chosen not to create any.

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.

On 26 June 2008, ICANN approved the relaxation of the rules for the introduction of new TLDs. The new rules will allow any public or private entity from anywhere in the world to register any string of letters as a gTLD. Observers believed that the new rule could result in hundreds of new gTLDs registered this year.

On 30 July 2008, the U.S. Department of Commerce reiterated the statement that it has no plans to transition management of the authoritative root zone file to ICANN.

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, .oz, .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. One notable exception is the 2007 emergence of SWIFTNet Mail, which uses the .swift pseudo-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.

TLDs in alternative roots

ICANN's slow progress in creating new gTLDs, and the high registration costs associated with TLDs, contributed to the creation of alternate root servers with their own sets of TLDs. 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.

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.

References

• 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.

External links

• Articles on CircleID about TLDs
• IANA TLD List
• Top-Level Domain Names by Host Count. ISC. Retrieved on 2008-08-07..