A domain name “extension” is called a “top-level domain (TLD).” These are divided into three different categories: generic top-level domains (gTLD), country code top-level domains (ccTLD) and infrastructure top-level domains.

Generic top-level domains are the ones most frequently seen on the Web:

• .com (originally intended for use by commercial organizations but is available to anyone).
• .net (originally intended for use by sites directly related to the internet  but is available to anyone).
• .org (originally intended for use by non-profit organizations but is available to anyone).
• .edu (used by educational organizations).
• .gov (reserved for agencies of the United States government).
• .mil (reserved for the United States military).
• .int (reserved for international organizations established by treaty. i.e. the European Union: http://europa.eu.int).
• .aero (reserved for members of the air transport industry).
• .biz (for use by businesses only).
• .coop (reserved for cooperative associations).
• .info.
• .museum (reserved for museums).
• .name (reserved for individuals).
• .pro (being developed for professionals and related entities).

The next top-level domains are the two letter country codes. These are used to designate a country or a dependent territory. For instance, in the URL http://bsnl.in/, the “in” represents the India. A listing of all country codes can be found on the IANA Web site. IANA (Internet Assigned Numbers Authority) oversees the allocation of all TLD’s.

The top-level domain infrastructure is limited to “.arpa” and is used exclusively for Internet-infrastructure purposes. The .arpa designation comes from the United States Department of Defense Advanced Research Projects Agency, the government agency which formulated the Internet.