All ASCII ccTLD identifiers are two letters long, and all two-letter top-level domains are ccTLDs. In 2018, the Internet Assigned Numbers Authority (IANA) began implementing internationalized country code top-level domains, consisting of language-native characters when displayed in an end-user application. Creation and delegation of ccTLDs is described in RFC 1591, corresponding to ISO 3166-1 alpha-2 country codes. While gTLD have to obey international regulations, ccTLD are subjected to requirements that are determined by each country’s domain name regulation corporation. With over 150 million domain name registrations today, ccTLDs make up 40% of the total domain name industry.  Country code extension applications began in 1985. The registered first extensions that year were .us (United States), .uk (United Kingdom), and .il (Israel).  There are 312 ccTLDs in active use totally. .cn, .tk, .de and .uk contain the highest number of domains.  The most expensive ccTLD sale ever recorded is kredit.de. It was sold 1,169,175 USD on 12 February 2008.  The second one was cruises.co.uk, which was sold 1,099,798 USD on 12/02/08.  The third most expensive sale among ccTLD was cruise.co.uk. It is the singular form of the second domain and was sold for 1,099,798 USD on 06/02/08. 
As of 2015, IANA distinguishes the following groups of top-level domains:
IANA is responsible for determining an appropriate trustee for each ccTLD. Administration and control are then delegated to that trustee, which is responsible for the policies and operation of the domain. The current delegation can be determined from IANA's list of ccTLDs. Individual ccTLDs may have varying requirements and fees for registering subdomains. There may be a local-presence requirement (for instance, citizenship or other connection to the ccTLD), as, for example, the Canadian (ca) and German (de) domains, or registration may be open.
The first registered ccTLD is .no, which was registered in 1983. Later ccTLDs are registered were .us, .uk, and .il in 1985, then followed by .au, .de, .fi, .fr, .jp, .kr, .nl, and .se were also registered in 1986.
The IANA is not in the business of deciding what is and what is not a country. The selection of the ISO 3166 list as a basis for country code top-level domain names was made with the knowledge that ISO has a procedure for determining which entities should be and should not be on that list.
Almost all current ISO 3166-1 codes have been assigned and do exist in DNS.
However, some of these are effectively unused. In particular, the ccTLDs for the Norwegian dependency Bouvet Island (
bv) and the designation Svalbard and Jan Mayen (
sj) do exist in DNS, but no subdomains have been assigned, and it is Norid policy to not assign any at present. Two French territories—
bl (Saint Barthélemy) and
mf (Saint Martin)—still[update] await local assignment by France's government.
eh, although eligible as ccTLD for Western Sahara, has never been assigned and does not exist in DNS. Only one subdomain is still registered in
gb (ISO 3166-1 for the United Kingdom), and no new registrations are being accepted for it. Sites in the United Kingdom generally use
uk (see below).
The former .um ccTLD for the U.S. Minor Outlying Islands was removed in April 2008. Under RFC 1591 rules, .um is eligible as a ccTLD on request by the relevant governmental agency and local Internet user community.
Several ASCII ccTLDs are in use that are not ISO 3166-1 two-letter codes. Some of these codes were specified in older versions of the ISO list.
uk(United Kingdom): The ISO 3166-1 code for the United Kingdom is GB. However, the JANET network had already selected
ukas a top-level identifier for its pre-existing Name Registration Scheme, and this was incorporated into the DNS root.
gbwas assigned with the intention of a transition, but this never occurred and the use of
ukis now entrenched.
suThis obsolete ISO 3166 code for the Soviet Union was assigned when the Soviet Union was still extant; moreover, new
suregistrations are accepted.
ac(Ascension Island): This code is a vestige of 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, with Ascension Island now the sole outlier. (Three other ccTLDs,
im(Isle of Man) and
je(Jersey) also fell under this category from 1996 until they received corresponding ISO 3166 codes in March 2006.)
eu(European Union): On 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. Only EU currently meets this criterion. Following a decision by the EU's Council of Telecommunications Ministers in March 2002, progress was slow, but a registry (named EURid) was chosen by the European Commission, and criteria for allocation set: ICANN approved
euas a ccTLD, and it opened for registration on 7 December 2005 for the holders of prior rights. Since 7 April 2006, registration is open to all in the European Economic Area.
ccTLDs may be removed if that country ceases to exist. There are three ccTLDs that have been deleted after the corresponding 2-letter code was withdrawn from ISO 3166-1:
cs (for Czechoslovakia),
zr (for Zaire) and
tp (for East Timor). There may be 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. Other ccTLDs corresponding to obsolete ISO 3166-1 codes have not yet been deleted. In some cases they may never be deleted due to the amount of disruption this would cause for a heavily used ccTLD. In particular, the Soviet Union's ccTLD
su remains in use more than twenty years after SU was removed from ISO 3166-1.
The temporary reassignment of country code
cs (Serbia and Montenegro) until its split into
me (Serbia and Montenegro, respectively) led to some controversies about the stability of ISO 3166-1 country codes, resulting in a second edition of ISO 3166-1 in 2007 with a guarantee that retired codes will not be reassigned for at least 50 years, and the replacement of RFC 3066 by RFC 4646 for country codes used in language tags in 2006.
The previous ISO 3166-1 code for Yugoslavia, YU, was removed by ISO on 2003-07-23, but the
yu ccTLD remained in operation. Finally, after a two-year transition to Serbian
rs and Montenegrin
me, the .yu domain was phased out in March 2010.
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 (.中国). 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.
ICANN requires all potential international TLDs to use at least one letter that does not resemble a Latin letter, or have at least three letters, in an effort to avoid IDN homograph attacks. Nor shall the international domain name look like another domain name, even if they have different alphabets. Between Cyrillic and Greek alphabets, for example, this could happen.
Lenient registration restrictions on certain ccTLDs have resulted in various domain hacks. Domain names such as
go.to form well-known English phrases, whereas others combine the second-level domain and ccTLD to form one word or one title, creating domains such as
blo.gs of South Georgia and the South Sandwich Islands (
youtu.be of Belgium (
del.icio.us of the United States (
cr.yp.to of Tonga (
.co domain of Colombia has been cited since 2010 as a potential competitor to generic TLDs for commercial use, because it may be an abbreviation for company.
Several ccTLDs allow the creation of emoji domains.
Some of the world's smallest countries and non-sovereign or colonial entities with their own country codes have opened their TLDs for worldwide commercial use, some of them free like .tk.
In 2014 a US court awarded a group of American and Israeli victims of Iranian terror the rights to the .ir domain, the suffix used to identify Iranian websites, along with Iran’s IP addresses being utilized by the Iranian government and its agencies.
Linkene går til registreringsenheten for det enkelte toppdomene [Links to the registration unit for each top level domain]