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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 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.
2.2.1 Top-Level Domains
The
original
top-level
domains divided the Internet domain namespace 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 U.C. 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 has been open to any commercial
organization, like com is.
org
Formerly
noncommercial organizations, such as the Electronic Frontier
Foundation (eff.org). Like
net, though, 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 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 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
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.
2.2.1.1 Country-code top-level domains
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.[3] 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.[3] 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.
2.2.1.2 New top-level domains
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.[4] These new gTLDs are:[4] ICANN refers to this latter variety as a
"sponsored gTLD," and the former as
an "unsponsored gTLD."
aero
For the aeronautical industry
biz
Generic
coop
For cooperatives
info
Generic
museum
For museums
name
For individuals
pro
For professionals
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 wasn't
originally true of the us top-level domain,
though. In the beginning, the us domain had 50
subdomains that correspond toguess what?the 50 U.S.
states.[5] Each was 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 was still largely
geographical: most subdomains corresponded to individual cities.
Beneath the cities, the subdomains usually corresponded to individual
hosts.[5] 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 uslike
com and netis open
to all comers.
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 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
domainand 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 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.
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 inthat 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 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.
