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Lesson 3 Top-level domains
Objective Define top-level subdomains of the DNS.

Top-level Domains

Under the current organization of the Domain Name Service (which may change soon), the root domain is subdivided into a set of top-level subdomains. These subdomains are intended to contain all hosts belonging to organizations of particular types. They are described in the table below.

Top-level Domain Description
arpa ARPAnet domain (now obsolete)
com Commercial organizations
edu Educational organizations
gov Civilian government organizations
mil Military organizations
net Network support centers
org Other organizations (non-profits, lobbying groups, Political Action Committees)
int International organizations
country code Geographic code for each country;
for example:
us=United States
de=Germany
uk=United Kingdom
il=Israel

Below these top-level domain names, the managers of the DNS delegate further subdivision of the DNS namespace to organizations with networks connected to the Internet.
This delegation takes place through the process of domain name registration, in which an organization registers its chosen name and associated network addresses with the InterNIC and its agent, the private company Network Solutions, Inc.
Once an organization registers its domain name (such as acme.com or harvard.edu), it is free to further subdivide that name. Thus the owners of the acme.com domain name may then freely create sales.acme.com, development.acme.com, and so forth.

Delegation

All name servers read the identities of the root servers from a local config file or have them built into the code. The root servers know the name servers for com, net, edu, fi, de, and other top-level domains. Farther down the chain, edu knows about colorado.edu, berkeley.edu, and so on. Each domain can delegate authority for its subdomains to other servers. Let us inspect a real example. Suppose we want to look up the address for the machine vangogh.cs.berkeley.edu from the machine lair.cs.colorado.edu. The host lair asks its local name server, ns.cs.colorado.edu, to figure out the answer.