2.2. The Internet Domain Name Space
So 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 Domains
The original top-level domains divided
the Internet domain name space organizationally into seven
domains:
- com
- Commercial
organizations, such as Hewlett-Packard (hp.com),
Sun Microsystems (sun.com), and IBM
(ibm.com).
- edu
- Educational
organizations, such as UC Berkeley
(berkeley.edu) and Purdue University
(purdue.edu).
- gov
- Government
organizations, such as NASA (nasa.gov) and the
National Science Foundation (nsf.gov).
- mil
- Military
organizations, such as the U.S. Army (army.mil )
and Navy (navy.mil ).
- net
- Formerly,
organizations providing network infrastructure, such as NSFNET
(nsf.net) and UUNET
(uu.net). Since 1996,
however, net, like
com, has been open to any
commercial organization.
- org
- Formerly,
noncommercial organizations, such as the Electronic Frontier
Foundation (eff.org ). Like net,
however, restrictions on org were
removed in 1996.
- int
- 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 domain names 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: 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.[10] 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".
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, however. The us domain has 50 subdomains
that correspond to -- guess what? -- the 50
states.[11] 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 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:
- lithium.cchem.berkeley.edu
- You've got a head start on this one, as we've already
told you that berkeley.edu is UC
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.
- winnie.corp.hp.com
- 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 mentioned 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.
- fernwood.mpk.ca.us
- 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.)
- daphne.ch.apollo.hp.com
- We've included this example just so you don't start
thinking that all domain names have four labels.
apollo.hp.com is the former Apollo
Computer's subdomain of the hp.com domain.
(When HP acquired Apollo, it also acquired Apollo's Internet
domain, apollo.com, which later became
apollo.hp.com.)
ch.apollo.hp.com is Apollo's Chelmsford,
Massachusetts, site. And daphne is a host in
Chelmsford.
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