Here are the words of Jon Postel himself from RFC 1591:
In the Domain Name System (DNS) naming of
computers 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.
Under
each TLD may be created a hierarchy of names. Generally, under the generic
TLDs the structure is very flat. That is, many
organizations are registered
directly under the TLD, and any further
structure is up to the individual organizations.
In the country TLDs, there is a wide variation in the structure, in
some countries the structure is very flat, in others there is
substantial
structural organization. In some country domains the
second
levels are generic categories (such as, AC, CO, GO, and RE),
in others
they are based on political geography, and in still
others,organization
names are listed directly under the country
code. The organization for the
US country domain is described in RFC 1480
RFC 1591 Domain
Name System Structure and Delegation March 1994
Each of the generic TLDs was created for a general category of
organizations.
The country code domains (for example, FR, NL, KR,
US) are each organized
by an administrator for that country. These
administrators
may further delegate the management of portions of
the naming tree.
These administrators are performing a public service on behalf of the Internet community.
Descriptions of the generic domains and the US country domain follow.
Of these generic domains, five are international in nature, and two
are restricted to use by entities in the United States.
World Wide
Generic Domains:
COM - This domain is intended for commercial entities,
that is
companies. This domain has grown
very large and there is
concern about the administrative
load and system performance if the current growth pattern is continued. Consideration
is
being taken to subdivide the COM domain and
only allow future
commercial registrations in
the subdomains.
EDU - This domain was originally intended for all
educational
institutions. Many Universities,
colleges, schools,
educational service organizations,
and educational consortia
have registered here.
More recently a decision has been taken to limit further registrations to 4 year
colleges and
universities. Schools and
2-year colleges will be registered
in the country
domains (see US Domain, especially K12 and CC,
below).
NET - This domain is intended to hold only the computers of
network
providers, that is the NIC and NOC computers,
the
administrative computers, and the network
node computers. The customers of the network provider would have domain names
of
their own (not in the NET TLD).
ORG - This domain is intended as the miscellaneous TLD for
organizations that didn't fit anywhere else. Some non-
government organizations may fit here.
INT - This domain is for organizations established by international
treaties, or international databases.
United States
Only Generic Domains:
GOV - This domain was originally intended for
any kind of government office or agency. More recently a decision was taken
to
register only agencies of the US Federal
government in this
domain. State and local
agencies are registered in the
country