A top-level domain (TLD) is the last segment of the domain name. The TLD is the letters immediately following the final dot in an Internet address.
A TLD identifies something about the website associated with it, such as its purpose, the organization that owns it or the geographical area where it originates. Each TLD has a separate registry managed by a designated organization under the direction of the Internet Corporation for Assigned Names and Numbers (ICANN).
In our Internet address, http://whatis.techtarget.com: com is the top-level domain name; techtarget.com is the second-level domain name; and whatis is a subdomain name. All together, these constitute a fully-qualified domain name (FQDN); the addition of HTTP:// makes an FQDN a complete URL.
ICANN identifies the following categories of TLDs:
- Country-code top-level domains (ccTLD) -- Each ccTLD identifies a particular country and is two letters long. The ccTLD for the United States, for example, is .us
- Infrastructure top-level domain -- There is only one TLD in this group, ARPA (Address and Routing Parameter Area). The Internet Assigned Numbers Authority (IANA) manages this TLD for the IETF.
- Sponsored top-level domains (sTLD): These are overseen by private organizations.
- Generic top-level domains (gTLD) -- These are the most common and familiar TLDs. Examples include "com" for "commercial" and "edu" for "educational." Most gTLDs are open for registration by anyone, but there is also a subgroup that is more strictly controlled.
In April 2009, ICANN proposed an expansion of the TLD system to allow anyone to register and reserve any unused letter sequence as a TLD for their exclusive use. A company that sold software, for example, might like to use .soft as a TLD. According to ICANN chief executive Paul Levins, such an expansion could lead to thousands of new TLDs in the next few years.
Learn More About IT:
> Wikipedia has an entry about top-level domains.
> ICAAN maintains a list of top-level domains.
> Shawn McCarthy discusses issues around top-level domain expansion.