TCP/IP Guide

IP Subnet Identifiers, Subnet Addresses, and Host Addresses

The main advantage that conventional classful addressing without subnets offers over subnets is simplicity. For example, even though there can be problems with managing thousands of devices in a single Class B network, it is simple to assign addresses within the network: They are all lumped together, so any combination of bits can be used within the host ID (except for all-zeros and all-ones).

When you subnet, however, you create a two-level structure within the classful host ID: subnet ID and host ID. This means you must choose IP addresses for devices more carefully. In theory, you are selecting subnets to correspond to the physical networks within the organization, so you want to assign ...