2.2 The Internet Domain Namespace
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.
2.2.1 Top-Level Domains
The original top-level domains divided the Internet domain namespace organizationally into seven domains:
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 the examples: we've used primarily U.S.-based organizations. That's easier to understandand forgivewhen 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 domains are called generic top-level domains , or gTLDs. By the time you read this, we may have a few more of these, such as biz , pro , info , and name , to accommodate the rapid expansion of the Internet and the need for more domain name "space." The organization now responsible for management of the Domain Name System, the Internet Corporation for Assigned Names and Numbers, or ICANN, has decided to add seven new gTLDs. For information on their work, see http://www.icann.org. We'll talk more about ICANN later in this chapter.
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 D.
2.2.2 Further Down
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's not true of the us top-level domain, though. The us domain has 50 subdomains that correspond toguess what?the 50 U.S. states.  Each is named according to the standard two-letter abbreviation for the statethe same abbreviation standardized by the U.S. Postal Service. Within each state's domain, the organization is still largely geographical: most subdomains correspond to individual cities. Beneath the cities, the subdomains usually correspond to individual hosts.
2.2.3 Reading Domain Names
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: