Using One Network with Multiple Subnets

Using One Network with Multiple Subnets

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As in Figure 10-8, the design in Figure 10-9 requires three groups of IP addresses. Unlike Figure 10-8, this figure uses three subnets, each of which is a subnet of a single Class B network (network 150.1.0.0). Notice from Figure 10-9 that each subnet is much smaller than the original Class B network, but there are lots of subnets.

IP network numbers represent networks, and similarly, IP subnet numbers represent each subnet. The subnet number has the same value in the first part of the number as all the host addresses, and 0s in the last part. For instance, 150.1.1.0 is one of the subnet numbers in Figure 10-9. All addresses in the subnet begin with 150.1.1, so the subnet number includes those numbers as well. Because the fourth octet can be any valid number, the subnet number is simply 0 in the last octet.

Using subnets in the Figure 10-9 network saves IP addresses. This same internetwork of three Ethernet LANs only uses a part of Class B network 150.1.0.0, as opposed to the internetwork in Figure 10-8, which fully uses three Class B networks (150.1.0.0, 150.2.0.0, and 150.3.0.0). Also, if you add another LAN, instead of needing a brand new Class A, B, or C network, you have lots of unused subnets such as 150.1.4.0, 150.1.5.0, and so on.

This example shows just one way to use subnetting. You can subnet in many ways, including subnetting Class A, Class B, and even Class C networks. For now, you understand the basic concepts; just be aware that IP subnetting can get a fair amount more complicated than what's covered here.