top-level domain (TLD) definition

Contributor(s): Gerald Holowicki and Stefan Youngs

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, com is the top-level domain name; 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.


This was first published in April 2009

Dig Deeper on Application servers and Web service platforms



Find more PRO+ content and other member only offers, here.

1 comment


Forgot Password?

No problem! Submit your e-mail address below. We'll send you an email containing your password.

Your password has been sent to:


File Extensions and File Formats

Powered by: