Chapter 15. Analyzing Existing Subnet Masks

When you examine an existing subnet design, you can learn a lot about the subnetting design by analyzing the subnet mask. First, the mask divides addresses into two parts: prefix and host. Then, the network class further subdivides the addresses, breaking the prefix part into the network and subnet parts.

Once you know the size of the three parts of an IP address, you can make some generalizations about the subnet and the entire classful network. This chapter helps you reverse-engineer some of the design choices made when someone else chose a particular mask. To do so, this chapter examines the process of breaking IP addresses into the three parts (network, subnet, and host), based on the class and the subnet mask, along with the additional facts that can then be calculated based on that information.