TCP/IP Guide

IP Default Subnet Masks for Address Classes A, B, and C

In order to better understand how subnets divide a Class A, B, or C network, let's look at how the Class A, B, and C networks are represented in a subnetted environment. This might seem unnecessary if you aren't planning to create subnets, but the fact is, once subnetting became popular, most operating systems, networking hardware, and software assumed that subnetting would be used. Even if you decide not to subnet, you may need to express your unsubnetted network using a subnet mask.

In essence, a nonsubnetted Class A, B, or C network can be considered the default for the more general, custom-subnetted network. You can think of a nonsubnetted network as being the case where you choose to ...