The Solution: Subnetting Saves IP Host Addresses

The Solution: Subnetting Saves IP Host Addresses

IP subnetting relaxes the rules a little bit. Without subnetting, the following rules apply:

These two rules might have been somewhat intuitive to you, based on the earlier examples. However, to make sure routing works well, in Figure 10-8, all addresses that start with 150.1 need to be on the left-side Ethernetthat's Rule 1. Hannah and Dean, in networks 150.1.0.0 and 150.2.0.0, must be separated from each other by a routerin this case, R1. That's essentially Rule 2.

Subnetting allows the network engineer to subdivide a Class A, B, or C network into smaller piecescalled subnetsand treat each subnet by the old rules used for networks. So, to subnet a network and reduce IP address waste, the rules are changed a little:

This concept is better explained with an example. In Class B network 150.1.0.0, all hosts whose addresses begin with 150.1 are in the same network. An IP network is just a group of hosts with some part of their IP addresses holding the same value. Subnetting allows the network engineer to configure the network devices such that they think that the first 3 octets of the addresses identify the network. In this one example of subnetting, the network engineer can create the following subnets:

Although all the addresses are still in Class B network 150.1.0.0, the networking devices will not think of the network as one big group, but instead, as a lot of smaller groups, called subnets. A subnet is just a subdivision of a larger Class A, B, or C network. The term subnetting refers to the process whereby the engineer decides to create subnets. Figure 10-9 shows this same network diagram, now with subnetting implemented.