The Internet's domain-name system (DNS) allows users to refer to web sites and other resources using easier-to-remember domain names (such as "www.icann.org") rather than the all-numeric IP addresses (such as "126.96.36.199") assigned to each computer on the Internet. Each domain name is made up of a series of character strings (called "labels") separated by dots. The right-most label in a domain name is referred to as its "top-level domain" (TLD).
The DNS forms a tree-like hierarchy. Each TLD includes many second-level domains (such as "icann" in "www.icann.org"); each second-level domain can include a number of third-level domains ("www" in "www.icann.org"), and so on.
The responsibility for operating each TLD (including maintaining a registry of the second-level domains within the TLD) is delegated to a particular organization. These organizations are referred to as "registry operators", "sponsors", or simply "delegees."
There are several types of TLDs within the DNS:
- TLDs with two letters (such as .de, .mx, and .jp) have been established for over 250 countries and external territories and are referred to as "country-code" TLDs or "ccTLDs". They are delegated to designated managers, who operate the ccTLDs according to local policies that are adapted to best meet the economic, cultural, linguistic, and legal circumstances of the country or territory involved. For more details, see the ccTLD web page on the IANA web site.
- Most TLDs with three or more characters are referred to as "generic" TLDs, or "gTLDs". They can be subdivided into two types, "sponsored" TLDs (sTLDs) and "unsponsored TLDs (uTLDs), as described in more detail below.
- In addition to gTLDs and ccTLDs, there is one special TLD, .arpa, which is used for technical infrastructure purposes. ICANN administers the .arpa TLD in cooperation with the Internet technical community under the guidance of the Internet Architecture Board.