Setting up subnets

For administrative or technical reasons, many organizations choose to divide one network into several subnets. Subnetworking enables several local networks to appear as a single Internet network to off-site hosts. It divides the addresses for a single network to accommodate the fact that the network consists of several physical networks. You should consider using subnets in the following instances:

• When you want to hide the local network topology from the outside world. Using subnets requires only a single route to external gateways.

• When you want the ability to administer IP addresses locally. For example, a company may have an engineering subnet, a product marketing subnet, and a sales subnet, each administered by a different administrator who has control of IP addresses in a given range.

• When network bandwidth is limited due to cabling constraints. Setting up subnets, each separated by a gateway host, limits local subnet packets to those that are either destined for or sent from a local host. In this way, the overall network traffic each host on the subnet sees is lessened.

Setting up a subnet consists of:

• determining the appropriate IP addresses for your subnets

• configuring subnet hosts with the correct IP addresses and netmasks

• configuring gateway hosts between subnets

Routing can get complicated as the number of networks grows. For example, a small organization might give each local network a class C number. As the organization grows, administering network numbers may get out of hand. A better idea is to allocate a class B network for the entire company. Then, divide the class B network into physical networks using subnets. In this way, you can isolate hosts from changes you might make to the network in remote parts of the organization.

Subnets allow you more flexibility when assigning network addresses. The Internet Protocol allows 127 class A networks with 24-bit host fields; 16,383 class B networks with 16-bit host fields; and over 2 million class C networks with 8-bit host fields.