Columbia University
Information Technology

Domain System
Networks | Definitions and Terminology  | FTPTelnet | E-mail

The Internet standard for naming computers is called the "domain system." This hierarchical system references values such as country, type of organization, organization name, division name, and computer name. Below is an example: 

[email protected] 

The information in a mail address becomes more global as you read from left to right. The user's name is always to the left of an @ sign. Computer and organization names are always to the right. In the example above, the person, you, receives her mail on a computer called "cunix" at Columbia. Because Columbia is an educational organization, it is included in the top-level domain "edu". Other top-level domains are listed below: 
com commercial
gov government
mil military 
org nonprofit organization
net network operation and info centers

Outside of the U.S., top-level domains are two-letter country codes such as these:

au Australia
il Israel
jp Japan
uk United Kingdom


Internet Address

A thirty-two-bit number that uniquely identifies an Internet host. This address is typically represented in eight-bit numbers (octets) separated by dots, e.g., An Internet address consists of a network number and a host number, and may be a class A, B, or C address. A class A network address is formatted as N.H.H.H, providing seven bits of network number and twenty-four bits of host number (e.g., indicates host 117 on net 26). A Class B network address is formatted as N.N.H.H, providing fourteen bits of network number and sixteen bits of host number (e.g., indicates host 1.132 on net number 128.89). A Class C network address is formatted as N.N.N.H, providing twenty-two bits of network number and eight bits of host address (e.g., indicates host 28 on network number 192.1.14). 

The Internet is the interconnection of many networks throughout the world that speak the same language, namely the TCP/IP protocol suite. Internet with a capital I refers specifically to that internet that contains NSFNET, MILNET, and DDN.