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 given level of the namespace. 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, such as NASA (nasa.gov) and the National Science Foundation (nsf.gov).
Military organizations, such as the U.S. Army (army.mil) and Navy (navy.mil).
Formerly organizations providing network infrastructure, such as NSFNET (nsf.net) and UUNET (uu.net). Since 1996, however, net has been open to any commercial organization, like com is.
Formerly noncommercial organizations, such as the Electronic Frontier Foundation (eff.org). Like net, though, restrictions on org were removed in 1996.
International organizations, such as NATO (nato.int).
Another top-level domain called arpa was originally used during the ARPAnet's transition from host tables to DNS. All ARPAnet hosts originally had hostnames under arpa so they were easy to find. Later, they moved into various subdomains of the organizational top-level domains. However, the arpa domain remains in use in a way you'll read about later.
You may notice a certain nationalistic prejudice in our examples: we've used primarily U.S.-based organizations. That's easier to understand?and forgive?when you remember that the Internet began as the ARPAnet, a U.S.-funded research project. No one anticipated the success of the ARPAnet, or that it would eventually become as international as the Internet is today.
Today, these original seven domains are called generic top-level domains or gTLDs. The "generic" contrasts them with the country-code top-level domains, which are specific to a particular country.
To accommodate the increasing internationalization of the Internet, the implementers of the Internet namespace compromised. Instead of insisting that all top-level domains describe organizational affiliation, they decided to allow geographical designations, too. New top-level domains were reserved (but not necessarily created) to correspond to individual countries. Their domain names followed an existing international standard called ISO 3166. ISO 3166 establishes official, two-letter abbreviations for every country in the world. We've included the current list of top-level domains as Appendix C.
 Except for Great Britain. According to ISO 3166 and Internet tradition, Great Britain's top-level domain name should be gb. Instead, most organizations in Great Britain and Northern Ireland (i.e., the United Kingdom) use the top-level domain name uk. They drive on the wrong side of the road, too.
Then, in late 2000, the organization responsible for the management of the Domain Name System, the Internet Corporation for Assigned Names and Numbers, or ICANN, created seven new generic top-level domains to accommodate the rapid expansion of the Internet and the need for more domain name "space." A few of these were truly generic top-level domains, like com, net, and org, while others were closer in purpose to gov and mil: reserved for use by a specific (and sometimes surprisingly small) community. These new gTLDs are:
 ICANN refers to this latter variety as a "sponsored gTLD," and the former as an "unsponsored gTLD."
For the aeronautical industry
Within these top-level domains, the traditions and the extent to which they are followed vary. Some of the ISO 3166 top-level domains closely follow the U.S.'s original organizational scheme. For example, Australia's top-level domain, au, has subdomains such as edu.au and com.au. Some other ISO 3166 top-level domains follow the uk domain's lead and have organizationally oriented subdomains such as co.uk for corporations and ac.uk for the academic community. In most cases, however, even these geographically oriented top-level domains are divided up organizationally.
That wasn't originally true of the us top-level domain, though. In the beginning, the us domain had 50 subdomains that correspond to?guess what??the 50 U.S. states. Each was named according to the standard two-letter abbreviation for the state?the same abbreviation standardized by the U.S. Postal Service. Within each state's domain, the organization was still largely geographical: most subdomains corresponded to individual cities. Beneath the cities, the subdomains usually corresponded to individual hosts.
 Actually, there are a few more domains under us: one for Washington, D.C., one for Guam, and so on.
As with so many namespace rules, though, this structure was abandoned when a new company, Neustar, began managing us in 2002. Now us?like com and net?is open to all comers.
Now that you know what most top-level domains represent and how their namespaces are structured, you'll probably find it much easier to make sense of most domain names. Let's dissect a few for practice:
You've got a head start on this one, as we've already told you that berkeley.edu is U.C. Berkeley's domain. (Even if you didn't already know that, though, you could have inferred that the name probably belongs to a U.S. university because it's in the top-level edu domain.) cchem is the College of Chemistry's subdomain of berkeley.edu. Finally, lithium is the name of a particular host in the domain?and probably one of about a hundred or so, if they've got one for every element.
This example is a bit harder, but not much. The hp.com domain in all likelihood belongs to the Hewlett-Packard Company (in fact, we gave you this earlier, too). Their corp subdomain is undoubtedly their corporate headquarters. And winnie is probably just some silly name someone thought up for a host.
Here you'll need to use your understanding of the us domain. ca.us is obviously California's domain, but mpk is anybody's guess. In this case, it would be hard to know that it's Menlo Park's domain unless you knew your San Francisco Bay Area geography. (And no, it's not the same Menlo Park that Edison lived in?that one's in New Jersey.)
We've included this example just so you don't start thinking that all domain names have only four labels. apollo.hp.com is the former Apollo Computer subdomain of the hp.com domain. (When HP acquired Apollo, it also acquired Apollo's Internet domain, apollo.com, which became apollo.hp.com.) ch.apollo.hp.com is Apollo's Chelmsford, Massachusetts site. daphne is a host at Chelmsford.