A domain name is entered as part of a Web site URL or an e-mail address (for example, www.usps.com). The domain name is a combination of an organization's unique name combined with a top level domain name (.com, .org, etc...). The name is included in a global Domain Name System, which informs the computer of exact IP address(es) with that name.
Using the United States Postal Service domain as an example:
IP Address: 56.0.0.0 - 56.255.255.255
URL: http://www.usps.com
Server name: www.usps.com
Domain name: usps.com
Subdomain: www
Second level domain: usps
Top level domain: com
In general, the IP address and the server name are interchangeable.