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In
the Domain Name System... there is a hierarchy of names.
The root of system is unnamed. There are a set of what
are called "top-level domain names" (TLDs). These are the
generic TLDs (EDU, COM, NET, ORG, GOV, MIL, and INT), and
the two letter country codes from ISO-3166. It is extremely
unlikely that any other TLDs will be created.
-- Jon
Postel; Domain Name System Structure and Delegation
; RFC
1591; March 1994.
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Internet domain names come in four main
types -- top-level domains, second-level domains, third-level domains,
and
country domains.
Internet domain names are the alphanumeric identifiers we use
to refer to hosts on the Internet, like "LivingInternet.com".
The structure of Internet domain names was first described in RFC
819, and their syntax was first described in RFC
822. You
can look up existing Internet domain names with the whois service,
and get your own domain name from an accredited
registrar.
The following subsections provide information on top-level
domains, second-level domains, third-level
domains, and country domains.
Top-level domains. Internet domain
names are organized by their levels, with the higher levels on
the right.
For example, for the domain "mail.twenty.net" the top-level domain
is "net", the second-level domain is "twenty.net", and the third-level
domain is "www.twenty.net".
Originally, the top-level Internet domains were associated with organizations,
mainly ".
arpa", ".
csnet", ".
bitnet",
and ".
uucp".
On January 28, 1986, coincidentally the same day the
Challenger
space shuttle exploded, those same four organizations met at
SRI and
agreed to restructure the domain name space into the following eight
subject-specific top-level domains, with technical administration
performed by
ISI:
| TLD |
Usage |
| .bitnet |
Used
for computers on the BITNET network. |
| .com |
Originally
stood for "commercial" to indicate
a site used for commercial purposes, but it has since become the
most well-known top-level Internet domain, and is now used for
any kind of site. |
| .int |
Used
by "International" sites, usually NATO
sites. |
| .edu |
Used for educational institutions like universities. |
| .gov |
Used for US Government sites. |
| .mil |
Used for US Military sites. |
| .net |
Originally intended for sites related to the
Internet itself, but now used for a wide variety of sites. |
| .org |
Originally
intended for non-commercial "organizations",
but now used for a wide variety of sites. Managed by the Internet
Society. |
In December 2002, after a long debate, ICANN approved the
following additional top-level Internet domains: .aero, .biz,
.coop, .info, .museum, .name, and .pro. In October, 2004,
ICANN
approved the top-level domains .travel and .post. Periodically
new top level domains continue to be approved.
Periodically there are proposals to establish a
special top
level Internet domain name for compartmentalization of adult material. RFC
3675, in the
words of the summary when it was released, "explains
why this is an ill considered idea from the
legal,
philosophical,
and particularly, the technical
points of view".
Second-level domains. Top-level
Internet domains like ".com" are shared by all the organizations
in the domain. Second-level domain names like "yahoo.com" and "livinginternet.com" are
registered by individuals and organizations. Second-level
domains are the addresses commonly used to host Internet
applications like web hosting and
email addressing.
Excluding the top-level domain portion, second-level domain
names can have up to 61 characters. For many years, character
were restricted to the 26 letters, 10 numbers,
or the hyphen character, except the hyphen can't be the first
or
last character.
Under these conditions, there are 36 possibilities for the
first and last character of the domain name, and 37 possibilities
for the other 59 characters. Therefore, the total number
of possible different second level domain names was:
37^59 x 36 x 36
=
433,018,769,429,481,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
You can see why people say there will always
be second-level domain names available. And since the introduction
of internationalization
of the Internet domain name system (see RFC
3490), the number of possibilities for each character
has increased considerably. Thanks to the
power of exponentiation, the number of different possible
second level domain names is now so large as to
be
effectively
infinite. However, only a few
Internet domains correspond to recognizable words, and the
vast majority
are
random
looking names like mebiwoplax234.com,
which not surprisingly was still available when last checked.
Third-level domains. Third-level
Internet domain names are created by those that own second-level
domains. Third-level domains can be used to set up individual
domains for specific purposes, such as a domain for web access
and
one for mail, or a separate site
for a special purpose:
- www.livinginternet.com
- mail.livinginternet.com
- rareorchids.livinginternet.com
Fourth-level and even higher Internet domains like "www2.un.news.media.twenty.net" can
be also be established. Three of four levels
is usually sufficient for most purposes.
Country domains. Each country
in the world has its own top-level Internet domain with a
unique alphabetic designation. A few countries and example
domains
are shown
below.
Organizations in each country are responsible for managing
the top-level Internet domain, and then for allocating second-level
domains within that domain to people and organizations with
interests in that country. The Country Code Domain Name Supporting
Organization (CCNSO)
is a support organization of ICANN responsible
for developing consensus
positions and recommending global policies relating to country-code
top-level domains. The following resources provide more information
about country domains
and organizations:
Historical references:
