Top-level domain
Internet domain names consist of parts separated by periods; the last part is the
top-level domain or
TLD. For example, in the domain name
wikipedia.org the top-level domain is
org (or
ORG, as domain names are not case-sensitive).
Two kinds of top-level domains exist. A
country code top-level domain (ccTLD) is used by a country or a dependent territory and is two letters long, for example
jp for
Japan). A
generic top-level domain (gTLD) is three or more letters long and is used (at least in theory) by a particular class of organizations (for example,
com for commercial organizations). 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.
See also:
List of Internet TLDs.
Country code top-level domains
\nThere are over 240 ccTLDs: see List of Internet TLDs and [1]. Most ccTLDs correspond to the two-letter ISO 3166-1 country codes, but there are several differences, explained below.
Each country appoints managers for its ccTLD and sets the rules for allocating domains. Some countries allow anyone in the world to acquire a domain in their ccTLD, for example Armenia (am), Austria (at) Cocos Islands (cc), Germany (de), Niue (nu), Samoa (ws), Tonga (to), Turkmenistan (tm) and Tuvalu (tv). This has resulted in the domain names I.am, start.at and go.to.
Other countries or dependent territories allow only residents to acquire a domain in their ccTLD, for example Canada (ca) and Mongolia (mn).
ISO 3166-1 codes not used as ccTLDs
\nThe 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.
The new codes tl (East Timor), cs (Serbia and Montenegro) and ax (Åland Islands) are not yet used as ccTLDs.
The ccTLDs for the Norwegian territories Bouvet Island (bv) and Svalbard (sj) do exist in DNS, but no subdomains have been assigned.
Very few (if any) sites use gb (United Kingdom) and no new registrations are being accepted for it. Sites in the UK use uk.
ccTLDs not in ISO 3166-1
\nEight ccTLDs currently remain in use despite not being ISO 3166-1 two-letter codes:\n*ac (Ascension Island), gg (Guernsey), im (Isle of Man) and je (Jersey): these codes came 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.\n*su (the obsolete ISO 3166-1 code for Soviet Union; the su managers stated in 2001 they will commence accepting new su registrations, but it is unclear whether this action is compatible with ICANN policy)\n*tp (the previous ISO 3166-1 code for East Timor)\n*uk (United Kingdom; this dates back to the early days of the Internet before the policy of using ISO 3166-1 codes had been settled)\n*yu (Serbia and Montenegro; the previous ISO 3166-1 code for Yugoslavia)
Other ccTLDs
\nOn September 25, 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.\nOnly eu (for the European Union) currently meets this criterion.\nFollowing a decision by the EU's Council of Telecommunications Ministers in March 2002, progress has been slow, but a registry (named EURid) has been appointed, and criteria for allocation set: the current estimate is that the eu ccTLD will be open for registrations in late 2004/early 2005.
Generic top-level domains
\nWhen top-level domains were first implemented, in January 1985, there were seven:\n*arpa — (see below)\n*com — commercial\n*edu — educational establishments (primarily US)\n*gov — US government\n*net — network infrastructure\n*org — other organizations not clearly falling within the other gTLDs\n*mil — US military
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 used for reverse DNS lookup for IPv4 addresses, so it has been retained for Internet-infrastructure purposes. The arpa TLD no longer has any connection with the ARPANET, 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. IANA considers arpa to be an infrastructure domain rather than a generic domain.
In November 1988, another gTLD was introduced:\n*int — international organizations established by treaty (although it is also used for some Internet infrastructure databases, such as ip6.int, the IPv6 equivalent of in-addr.arpa).\nThis TLD was introduced in response to NATO's request for a domain name which adequately reflected its character as an international organization -- see discussion of nato below. In May 2000, the Internet Architecture Board proposed to close the int domain to new infrastructure databases. All future such databases would be created in arpa, and existing ones would move to arpa wherever feasible.
By the mid-1990s there was pressure for more gTLDs to be introduced. Jon Postel, as head of IANA, invited applications from interested parties [1]. In early 1995, Postel created "Draft Postel", an Internet draft containing the procedures to create new domain name registries and new TLDs. Draft Postel created a number of small committees to approve the new TLDs. Because of the increasing interest, an number of large organizations took over the process under the Internet Society's umbrella. This second attempt involved the setting up of a temporary organization called the International Ad Hoc Committee (IAHC). On 4 February 1997, the IAHC issued a report ignoring the Draft Postel recommendations and instead recommended the introduction of seven new gTLDs (arts, firm, info, nom, rec, store and web). However, progress on this stalled after the US Government intervened and nothing ever came of it.
In October 1998, the Internet Corporation for Assigned Names and Numbers (ICANN) formed to take over the task of managing domain names. After a call for proposals (August 15, 2000) and a brief period of public consultation, ICANN announced on November 16, 2000 its selection of the following seven new gTLDs:
- aero — air transport industry\n*biz — businesses\n*coop — cooperatives\n*info — information (unrestricted use)\n*museum — museums\n*name — individuals\n*pro — professionals
These new gTLDs started to come into use in June
2001, and by the end of that year all except
pro existed, with
biz,
info and
museum already in full operation.
name and
coop became fully operational in January
2002, and
aero followed later in the year.
pro became a gTLD in May 2002.
ICANN now intends to add further gTLDs, starting with a set of sponsored top-level domains (like the current
aero,
coop and
museum). The application period for these lasted from
15 December 2003 until
16 March 2004, and resulted in ten applications. The most high-profile of these applications came from a consortium of companies including
Microsoft,
Vodafone,
Samsung,
Sun Microsystems and
Nokia. It aims to develop a gTLD for mobile devices, potentially offering stripped-down versions of existing sites. The full list of proposed new TLDs consists of:
asia,
cat (or
ctl or
catala),
jobs,
mail (or
tmail or
mta),
mobi (or
mbl),
post,
tel,
travel and
xxx. Two separate, unrelated entities applied for
tel.
Historical TLDs
\nThe 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 for example the
IP address 212.30.222.56 is mapped to a host name by issuing a DNS query for the PTR record for the special host name
56.222.30.212.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 a decade 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 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.
Reserved TLDs
\nRFC 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\n*invalid — reserved for use in obviously invalid domain names\n*localhost — reserved to avoid conflict with the traditional use of localhost\n*test — reserved for use in tests
TLDs in alternate roots
\nAlternate DNS roots have their own sets of TLDs. See that article for details.
See also
\n*List of Internet TLDs
References
\n*Addressing the World: National Identity and Internet Country Code Domains, edited by Erica Schlesinger Wass (Rowman & Littlefield, 2003, ISBN 0742528103) [1], examines connections between cultures and their ccTLDs.\n*
Ruling the Root by Milton Mueller (MIT Press, 2001, ISBN 0262134128)
[1], discusses TLDs and domain name policy more generally.
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