IP Addressing

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An IP (Internet Protocol) address is a unique identifier for a node or host connection on an IP network. An IP address is a 32 bit binary number usually represented as 4 decimal values, each representing 8 bits, in the range 0 to 255 (known as octets) separated by decimal points. This is known as "dotted decimal" notation.


It is sometimes useful to view the values in their binary form.

140     .179     .220     .200

Every IP address consists of two parts, one identifying the network and one identifying the node. The Class of the address and the subnet mask determine which part belongs to the network address and which part belongs to the node address.

Address Classes

There are 5 different address classes. You can determine which class any IP address is in by examining the first 4 bits of the IP address.

Addresses beginning with 01111111, or 127 decimal, are reserved for loopback and for internal testing on a local machine; [You can test this: you should always be able to ping, which points to yourself] Class D addresses are reserved for multicasting; Class E addresses are reserved for future use. They should not be used for host addresses.

Now we can see how the Class determines, by default, which part of the IP address belongs to the network (N, in blue) and which part belongs to the node (n, in red).

In the example, is a Class B address so by default the Network part of the address (also known as the Network Address) is defined by the first two octets (140.179.x.x) and the node part is defined by the last 2 octets (x.x.220.200).

In order to specify the network address for a given IP address, the node section is set to all "0"s. In our example, specifies the network address for When the node section is set to all "1"s, it specifies a broadcast that is sent to all hosts on the network. specifies the example broadcast address. Note that this is true regardless of the length of the node section.

Private Subnets

There are three IP network addresses reserved for private networks. The addresses are, Subnet Mask,, Subnet Mask, and, Subnet Mask These addresses are also notated,, and; this notation will be explained later in this tutorial. They can be used by anyone setting up internal IP networks, such as a lab or home LAN behind a NAT or proxy server or a router. It is always safe to use these because routers on the Internet by default will never forward packets coming from these addresses. These addresses are defined in RFC 1918.

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Updated January 29, 2007
Copyright © 1996-2007 by Ralph Becker < > send me Feedback!