2.2. The Internet Domain Name SpaceSo far, we've talked about the theoretical structure of the domain name space and what kind 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 doesn't attach any particular meaning to the labels at a particular level. When you manage a part of the domain name space, 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 name space, however, has some self-imposed structure to it. Especially in the upper-level domains, domain names follow certain traditions (not rules, really, as they can be and have been broken). These traditions help 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 DomainsThe original top-level domains divided the Internet domain name space organizationally into seven domains:
You may notice a certain nationalistic prejudice in the examples: all are primarily U.S. 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 domains are called generic top-level domains, or gTLDs. In early 2001, we will have a few more of these, including name, biz, info, and pro, to accommodate the rapid expansion of the Internet and the need for more domain name "space." The organization responsible for management of the Internet's domain name system, the Internet Corporation for Assigned Names and Numbers (ICANN) OK'd adding these new gTLDs, along with the decidedly nongeneric aero, coop, and museum, in late 2000. For information on ICANN's work and the new TLDs, see http://www.icann.org.
To accommodate the increasing internationalization of the Internet, the original 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, "Top-Level Domains".
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.
2.2.2. Further DownWithin 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, however. The us domain has 50 subdomains that correspond to -- guess what? -- the 50 states. Each is 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 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 NamesNow 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:
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