Understanding IP Addresses

At the root of any network running TCP/IP is a unique identifier known as an IP address. In the same way that your home is identified by a street name (more general) and numeric address (more specific), a computer on a TCP/IP network is identified by an IP address like Similar to your home address, part of an IP address identifies a particular network, and another part represents a unique computer (referred to as a host in TCP/IP jargon) on that network.

Because of the way in which hosts running TCP/IP identify and communicate with one another,
you cannot just blindly pick numbers to assign as IP addresses. In order for two computers on the same network to communicate, they must be assigned addresses that begin with the same network number, and then the host portion of each address must be unique. For example, if you were to give one computer an IP address of and another on the same network an IP address of, the two could never communicate – from their internal calculations, the two hosts would “see” each other as being on different networks, and would not attempt to communicate directly.

Distinguishing network numbers isn’t quite as easy as starting IP addresses with a common number like “192”. IP addresses are divided into what are known as “classes”, and the first number in an IP address is used to determine how much of the IP address is the network portion, and how much is the host portion. For example, when an IP address starts with the number 10, only the first section (known as an octet) represents the network, and the last three octets are used to identify a host. However, when an IP address starts with the number 213, the first three octets uniquely identify the network, and only the last octet identifies a host.

The reason for IP addresses being separated into different classes is to allow for networks of different sizes on the Internet – some networks are small, while others are much larger. The table below outlines the three main address classes used on the Internet. The value in the first octet of the address is used to determine the address class. For this information you can subsequently determine which portion of an IP address identifies a network, and which a host.

Address Class First Octet Range Network Portion (N) and Host Portion (H)
Class A 1-126 N.H.H.H
Class B 128-191 N.N.H.H
Class C 192-223 N.N.N.H

Looking at the table above, you would now know that the IP address is a Class B address, and that 167.204 identifies the network, while 7.1 uniquely identifies the host. So, if two hosts on the same network were configured with the IP addresses and respectively, they would be able to communicate using TCP/IP.

You may have noticed that the number “127” appears to be missing from the table above. It is, and this is on purpose. IP addresses starting with 127 are reserved for a special diagnostic function on a TCP/IP network, and are known as “loopback” addresses. When you ping the address, you are effectively pinging the same machine from which the command is issued. If you receive a reply, it means that TCP/IP is functioning on that computer. However, it still doesn’t mean that your configuration settings are correct.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of 2000Trainers.com. He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.