So far, we’ve talked about the theoretical structure of the domain namespace and what sort of data is stored in it, and we’ve even hinted at the types of names you might find in it with our (sometimes fictional) examples. But this won’t help you decode the domain names you see on a daily basis on the Internet.
The Domain Name System doesn’t impose many rules on the labels in domain names, and it doesn’t attach any particular meaning to the labels at a particular level. When you manage a part of the domain namespace, you can decide on your own semantics for your domain names. Heck, you could name your subdomains A through Z and no one would stop you (though they might strongly recommend against it).
The existing Internet domain namespace, however, has some self-imposed structure to it. Especially in the upper-level domains, the domain names follow certain traditions (not rules, really, as they can be and have been broken). These traditions help to keep domain names from appearing totally chaotic. Understanding these traditions is an enormous asset if you’re trying to decipher a domain name.
The original top-level domains divided the Internet domain namespace organizationally into seven domains:
Commercial organizations, such as Hewlett-Packard (hp.com), Sun Microsystems (sun.com), and IBM (ibm.com).
Educational organizations, such as U.C. Berkeley (berkeley.edu) and Purdue University (purdue.edu).
Government organizations, ...