Entry

What Are Top-Level Domains (TLDs)?

Jan 19th, 2008 06:20
dman, Laurent Chouinard, Kagan (Kai) MacTane, http://sturly.com

A Top-Level Domain (TLD) is one of the highest-level domains on the 
Internet, usually represented by the last two or three letters of an 
address, URL, or domain name. For example, consider the following 
addresses (some of which are hostnames, some of which are URLs, and 
some 
of which are email addresses):
   http://lsyg.com
   hlovecra@miskatonic.edu
   www.demon.co.uk
   well.sf.ca.us
   asus.com.tw
In the above list, the TLDs are, in order: .com, .edu, .uk, .us, and 
.tw. The lsyg.com domain is a subset of the .com TLD. The host 
www.demon.co.uk is a member of the domain demon.co.uk, which is a 
subset 
of the .co.uk domain, which in turn is a subset of the .uk TLD.
TLDs can be grouped into two major groups: _Geographic_ domains and 
_Organizational_ domains. These are also referred to as "country codes" 
and "role domains", since the geographic domains are based on national 
borders, and the organizational domains are based on the "role" an 
organization plays in the Uninted States.
The U.S. organizational domains are the following six, all containing 
three letters:
   .com   COMmercial entities: businesses, shops, merchants, etc.
   .edu   EDUcational institutions (accredited higher learning 
          centers only; public grade schools go under .gov)
   .org   Non-profit ORGanizations: charities, lobbying groups, 
          etc.
   .net   NETworking providers: ISPs; major access providers; 
          groups that maintain backbone routes and cables; etc.
   .gov   GOVernmental (non-military) bodies: Congress; state 
          governments; Cabinet departments such as the DoE, 
          DoD, HUD, etc.; NASA, the IRS, and similar stuff.
   .mil   MILitary divisions: the Army, Navy, Air Force, Marines,
          and so forth.
These categories used to be pretty firm. In recent years, however, the 
people responsible for registering the .com, .net, and .org domains 
(Network Solutions, Inc.) have been very lax abbout enforcing those 
standards, so nearly anyone can get a domain registered in any of those 
three TLDs. The .edu, .gov, and .mil TLDs, however, are still strictly 
controlled to ensure that only qualifying organizations can register 
domains within them.
Country codes, on the other hand, are far more numerous. They all use 
two-letter abbreviations, based on ISO 3166 (an international standard 
for abbreviating country names). Here are a few that are useful to know 
(please don't get angry at me if I didn't include your country!), and 
which will help give you the flavor of them:
   .au   Australia
   .ca   Canada
   .ch   Switzerland (Confederaziun Helvetica)
   .cn   China
   .de   Germany (Deutschland)
   .fr   France
   .ie   Ireland
   .il   Israel
   .it   Italy
   .jp   Japan
   .ro   Romania
   .ru   Russia
   .se   Sweden
   .uk   United Kingdom
   .us   United States
A complete list of country codes can be found at 
http://www.ics.uci.edu/pub/websoft/wwwstat/country-codes.txt, among 
other places on the Web.
There are a few countries which have elected to make domain names 
within 
their TLDs available to outsiders, generally for the purpose of setting 
up names that "look like standard English". Prominent among these are 
Tonga (.to), which has allowed servers such as go.to and come.to, 
Armenia (.am), for the i.am server, and Sao Tome and Principe (.st), 
allowing hosts like sesame.st, fascination.st, and the like.
Oddly, country code TLDs are sometimes more stable than the countries 
they represent! There is still a .su (Soviet Union) TLD in use on the 
Internet, because when the Soviet Union collapsed, there were some 
machines whose new ownership was unclear. Eventually, rather than 
assign 
them to, for instance, .ru, .by (Belarus) or .az (Azerbaijan), it was 
decided to simply leave them designated as being in the .su domain.
Finally, there are two "special" TLDs, each with four letters: .arpa 
and 
.nato. The .arpa domain stands for "Advanced Research Projects Agency", 
a division of the U.S. government that funded much of the early 
development of what eventually became the Internet. The .arpa TLD is 
mostly unused now, except for the in-addr.arpa domain, which is used to 
maintain a database of reverse lookups for DNS servers. (DNS, the 
Domain 
Name System, is used to translate host names into IP addresses and back 
again.)
The .nato domain is for use by NATO, the North Atlantic Treaty 
Organization.
There is also a .int Top-Level domain, intended for international 
organizations. Very few actually use the .int domain, as most companies 
either use a .com domain, or country-specific domains wherever they 
have businesses in. The Internet Assigned Numbers Authority is 
responsible for for .int TLD, and currently accepts registrations.
http://www.iana.org/int-dom/int.htm