Parts of the IPv4 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. This section describes IPv4 addresses. For information on IPv6 addresses, see "IPv6 Addressing".

The IPv4 address is a 32-bit number that uniquely identifies a network interface on a machine. An IPv4 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 IPv4 address. This form of representing the bytes of an IPv4 address is often referred to as the dotted-decimal format.

The bytes of the IPv4 address are further classified into two parts: the network part and the host part. The following figure shows the component parts of a typical IPv4 address, 129.144.50.56.

Figure 7-3 Parts of an IPv4 Address

Network Part

This part specifies the unique number assigned to your network. It also identifies the class of network assigned. In Figure 7-3, the network part takes up two bytes of the IPv4 address.

Host Part

This is the part of the IPv4 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 IPv4 address space by using some of the bits from the host number part of the IPv4 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 IPv4 address. (Refer to "Creating the Network Mask for IPv4 Addresses" for details.)