CCNA Discovery 2, Chapter 4
4.1.1–4.1.4: When you create a logical design for a network, you will likely use a 32-bit hierarchical IP addressing scheme. The size of your network and the need for host addresses will dictate the class of network you choose and the number of bits necessary for the network and host portions of your IP addresses. As soon as you have determined the potential need for hosts, you can identify the network portion of the addresses with a subnet mask. If you started with a Class A address, you can further divide the Class A network into multiple Class B networks by using a Class B subnet mask. Table 20-6 demonstrates how you might divide a Class A network into four smaller networks.