Top-level domain (TLD): overview, types, history and uses

The generic top-level domains (gTLD) are divided into sponsored (sTLD) and non-sponsored (uTLD). The (much more important) non-sponsored domains are under the direct control of ICANN and the Internet Society. The sponsored domains are controlled and funded by independent organizations. These organizations have the right to apply their own policies to the allocation of domain names. An example is .mil. This domain is used exclusively by the US military.

The most used TLD by far is .com (approx. 127 million registered domains in September 2017). It was originally used by US companies, but is now used worldwide.

Non-sponsored domains (uTLD)

Unsponsored top-level domains (uTLD) are used by a specific group. They consist of three or more characters and stand for a term that distinguishes this group.

Due to the liberal allocation for the TLD .com, .net, .org as well as (with minor restrictions) .biz and (more recently) .name, the original meanings of these TLDs have largely been lost. Such a TLD does not necessarily indicate a corresponding use. For example, the .org TLD, which was originally intended for non-commercial organisations, is now also occasionally used by commercial providers. Furthermore, international, non-commercial websites like to use .net (or .org) in order not to have to resort to a country code TLD or the widespread .com. While .org is now only listed as a generic TLD by IANA, the TLDs biz, .name and .pro are provisionally (as of August 2019) still classified as generic-restricted.

Sponsored Domains (sTLD)

Sponsored top-level domains (sTLDs) are proposed by certain companies or organizations that operate these namespaces according to detailed policies and also have control and sanction rights to ensure the intended and legal use of the registered names by the providers. For example, the .aero TLD is sponsored by SITA, which restricts its use to aviation content, or the use of .mobi names is conditional on the website provider ensuring compliance with certain guidelines that are considered elementary for device-independent use of web content, so that, for example, mobile phones can display this content.

New domains

On 26 June 2008, ICANN decided to relax the rules for new sponsored domains. In the following months, rules and the application process were developed for this. On 20 June 2011, ICANN adopted an application process for new generic top-level domains (gTLDs) in Singapore. Between 12 January 2012 and 30 May 2012, interested parties could apply for a new TLD. On June 13, 2012, known as Reveal Day, ICANN published a list of which new desired top level domains had applications submitted by which applicants. Over a period of about two years, ICANN planned to review the proposals and decide on any approvals. Finally, on October 23, 2013, the first four new top-level domains went live, with hundreds more top-level domains added over the next few years. Approved and used top-level domains of this new type are for example .berlin, .koeln and .swiss.

Special cases

There are some, mostly historical, special domains or pseudo-domains, as well as names reserved for specific purposes, for which TLDs are not established for various reasons:

There are over 200 ccTLDs (cc = country code), each country is assigned exactly one two-letter code (ALPHA-2) according to ISO 3166. In addition, there are often separate ccTLDs for dependent territories, which are usually geographically separated from the mother country.

Exceptions

• The United Kingdom uses the TLD .uk, although according to ISO its encoding is GB (GB,GBR,826). In addition, however, the ISO has also exceptionally reserved the encoding UK. The TLD .gb is also reserved, but is currently probably not used except for a single registration.
• The European Union uses a ccTLD with .eu, although it is not an independent state. However, the encoding EU was exceptionally reserved with this meaning by the ISO 3166 Maintenance agency. This was done by special decision due to an identified practical need. Unlike states, however, the EU has neither a three-letter nor a three-digit encoding. Its member countries will continue to retain their own ccTLDs, and no country code subdomains of .eu will be introduced.

Changes

• .dd was intended for the GDR, but was used exclusively internally at the universities in Jena and Dresden. There was no DNS delegation in the root name servers of the IANA, the top-level domain .dd was replaced by .de of the Federal Republic of Germany after the reunification.
• .cs was formerly used for Czechoslovakia. This TLD was later intended for the now dissolved confederation of Serbia and Montenegro. However, due to name conflicts with still existing websites and email addresses of the former Czechoslovakia, this was apparently not implemented on the servers. Instead, .yu continued to be used for Serbia and Montenegro. With the breakup into the individual states and their domains .rs and .me, the matter has been resolved. In the future, .cs will no longer be needed. The abbreviations used after the separation of Czechoslovakia are .cz for the Czech Republic and .sk for Slovakia.
• .zr for Zaire was removed from the root servers in 2004 (now .cd).
• .um for the United States Minor Outlying Islands was assigned to the registrar and operator University of Southern California Information Sciences Institute but never used. Therefore, in consultation with ICANN, the United States Department of Commerce - National Telecommunications and Information Administration (NTIA) and the operator, the .um domain was reset in an unoccupied status in 2008. In doing so, it was agreed that ICANN should not take any action regarding the future use of the .um domain without consulting with and obtaining prior approval from the U.S. government. Since then, .um has been listed by IANA in the Root Zone Database as "Not assigned" until today (2019).
• .yu for Yugoslavia was discontinued on 30 September 2009 (no longer accessible since 30 March 2010), as it was replaced by the two domain extensions .rs for Serbia and .me for Montenegro.
• .an for Netherlands Antilles was permanently deleted on 31 July 2015 due to its dissolution in 2010. .an was replaced by the inclusion of the top-level domains .bq (for the three "island territories" Bonaire, Sint Eustatius and Saba, which have become the Special Municipalities of the Netherlands) as well as .cw for Curaçao and .sx for Sint Maarten (both Caribbean islands now autonomous countries within the Kingdom of the Netherlands) in the root zone by ICANN on 20 December 2010.

Outgoing

Some obsolete TLDs are still active for accessibility reasons:

• In Russia, .su (former Soviet Union) is operated in addition to .ru. New domains are also registered under .su after the dissolution of the Soviet Union. This also includes presences from the German Rhein-Sieg district (vehicle registration number SU).
• East Timor switched from .tp to .tl and operated both TLDs for a transition period until 2015.

Unused

Allocated but unused at present are:

• .bq for the Caribbean Netherlands (Bonaire, Sint Eustatius and Saba)
• .eh for Western Sahara.

Not yet allocated

The following ccTLDs have not yet been assigned:

• .bl Saint-Barthélemy
• .mf Saint-Martin

Conditions of allocation

Each country has the right to set its own registry policies for its domain. In the vast majority of cases, these are drawn up independently by the registries on the basis of technical necessities and legal requirements and can differ considerably from one another.

For example, registration under the French TLD .fr originally required proof of a residence or place of business in France (in the European Union since 6 December 2011).

In Germany, the allocation of one-character and two-character domains as well as domains consisting only of digits with .de began on 23 October 2009. Until then, the .de domain had to consist of at least three characters, and one of them had to be a letter. From the early days of the Internet, there were three two-character domains: db.de, hq.de, and ix.de. (The two-letter domain bb.de, which was the fourth for a long time, was no longer registered at the time of the ruling). Volkswagen sued before the Frankfurt am Main Higher Regional Court for the allocation of the domain vw.de, which was then put into operation on the effective date.

In Switzerland, only the cantons have a domain with two characters, which is derived from the official abbreviation (e.g. ar.ch, ge.ch, ur.ch, zh.ch). In addition, the Federal Chancellery operates the domain ch.ch. Apart from these exceptions, Swiss domains must consist of at least three characters. Exceptions are www.expo.02.ch (for the sixth Swiss National Exhibition in 2002) and the domain au.ch of the municipality of Au in the canton of St. Gallen.

In other cases, only a few predefined names are possible for the second name level; the actual name is then defined as a third-level domain (e.g. example.co.uk). One example was the British .uk domain, which until 9 June 2014 only allowed the following second-level domains:

• .ac.uk - academic, educational institutions such as universities
• .co.uk - commercial
• .gov.uk - government, central and regional government organisations
• .ltd.uk - limited company
• .me.uk - presences of individuals
• .net.uk - ISPs and other network companies
• .nhs.uk - National Health Service, national health system and its institutions
• .nic.uk - Network Information Center, for internal network management only
• .org.uk - non-profit organizations
• .plc.uk - public limited companies (listed companies)
• .sch.uk - Schools

In addition, there are some government-used second-level domains such as .police.uk, .mod.uk (Ministry of Defence), .british-library.uk (as well as .bl.uk - also for the British Library) or .parliament.uk. These date from an earlier time and are grandfathered.