TCP/IP and Data Communications Administration Guide

Parts of the IP Address

Each network running TCP/IP must have a unique network number, and every machine on it must have a unique IP address. It is important to understand how IP addresses are constructed before you register your network and obtain its network number.

The IP address is a 32-bit number that uniquely identifies a network interface on a machine. An IP address is typically written in decimal digits, formatted as four 8-bit fields separated by periods. Each 8-bit field represents a byte of the IP address. This form of representing the bytes of an IP address is often referred to as the dotted-decimal format.

The bytes of the IP address are further classified into two parts: the network part and the host part. Figure 3-1 shows the component parts of a typical IP address,

Figure 3-1 Parts of an IP Address


Network Part

This part specifies the unique number assigned to your network. It also identifies the class of network assigned. In Figure 3-1, the network part takes up two bytes of the IP address.

Host Part

This is the part of the IP address that you assign to each host. It uniquely identifies this machine on your network. Note that for each host on your network, the network part of the address will be the same, but the host part must be different.

Subnet Number (Optional)

Local networks with large numbers of hosts are sometimes divided into subnets. If you choose to divide your network into subnets, you need to assign a subnet number for the subnet. You can maximize the efficiency of the IP address space by using some of the bits from the host number part of the IP address as a network identifier. When used as a network identifier, the specified part of the address becomes the subnet number. You create a subnet number by using a netmask, which is a bit mask that selects the network and subnet parts of an IP address. (Refer to "Creating the Network Mask" for full details.)