Subnet addresses

Subnet addressing allows an autonomous system made up of multiple networks to share the same Internet address.

The subnetwork capability of TCP/IP also makes it possible to divide a single network into multiple logical networks (subnets). For example, an organization can have a single Internet network address that is known to users outside the organization, yet it can configure its network internally into departmental subnets. In either case, fewer Internet network addresses are required while local routing capabilities are enhanced.

A standard Internet Protocol address field has two parts: a network address and a local address. To make subnets possible, the local address part of an Internet address is divided into a subnet number and a host number. The subnet is identified so that the local autonomous system can route messages reliably.

In the basic Class A Internet address, which consists of an 8-bit network address and 24-bit local address, the local address identifies the specific host machine on the network.

This illustration shows a typical class A address structure. The first 8 bits contain the network address (always beginning with a zero). The remaining 24 bits contain the local host address.

To create a subnet address for this Class A Internet address, the local address can be divided into a number identifying the physical network (or subnet) and a number identifying the host on the subnet. Senders route messages to the advertised network address, and the local system takes responsibility for routing messages to its subnets and their hosts. When deciding how to partition the local address into subnet address and host address, you should consider the number of subnets and the number of hosts on those subnets.

In the following figure, the local address is partitioned into a 12-bit subnet address and a 12-bit host address.

This illustration shows a typical class A address structure. The first 8 bits contain the network address (always beginning with a zero). The remaining 24 bits contain the local host address with the subnet address occupying the first 8 bits and the host address occupying the last 8 bits.

You have flexibility when assigning subnet addresses and host addresses. The bits of the local address can be divided according to the needs and potential growth of the organization and its network structure. The only restrictions are:

• network_address is the Internet address for the network.
• subnet_address is a field of a constant width for a given network.
• host_address is a field that is at least 1-bit wide.

If the width of the subnet_address field is 0, the network is not organized into subnets, and addressing to the network is performed using the Internet network address.

The bits that identify the subnet are specified by a bit mask and, therefore, are not required to be adjacent in the address. However, it is generally desirable for the subnet bits to be contiguous and located as the most significant bits of the local address.