The last and most significant component of an Internet fully qualified domain name, the part after the last ".". For example, host wombat.doc.ic.ac.uk is in top-level domain "uk" (for United Kingdom).

Every other country has its own top-level domain, including ".us" for the U.S.A. Within the .us domain, there are subdomains for the fifty states, each generally with a name identical to the state's postal abbreviation. These are rarely used however. Within the .uk domain, there is a .ac.uk subdomain for academic sites and a .co.uk domain for commercial ones. Other top-level domains may be divided up in similar ways.

In the US and some other countries, the following top-level domains are used much more widely than the country code:

        .com - commercial bodies
        .edu - educational institutions
        .gov - U. S. government
        .mil - U. S. armed services
        .net - network operators
        .org - other organisations

Since the rapid commercialisation of the Internet in the 1990s the ".com" domain has become particularly heavily populated with every company trying to register its company name as a subdomain of .com, e.g. "netscape.com" so as to make it easy for customers to guess or remember the URL of the comany's home page.

United Nations entities use the domain names of the countries where they are located. The UN headquarters facility in New York City, for example, is un.org.

Several new top-level domains are about to be added (Oct 1997):

        .nom   - individual people
        .rec   - recreational organisations
        .firm  - businesses such as law, accounting, engineering
        .store - commercial retail companies
        .ent   - entertainment facilities and organisations