Which description best summarizes Class A networks in classful IPv4 addressing?

• A. Few networks with many hosts per network
• B. Many networks with many hosts per network
• C. Many networks with few hosts per network
• D. Few networks with few hosts per network

Answer: (A)

In classful IPv4 addressing, networks are divided into classes with fixed default masks, and Class A uses a default mask of 255.0.0.0. This means the network portion is the entire first octet, giving only a limited number of networks (the 1 through 126 range; 0 and 127 are reserved). But each Class A network has a very large address space for hosts: 24 bits available for hosts, which yields 2^24 addresses per network (16,777,216 total addresses, minus the network and broadcast addresses, about 16,777,214 usable hosts). So you get a small number of networks, each capable of supporting a huge number of hosts. That’s why the best description is few networks with many hosts per network.

The other descriptions map to other classes: Class B would be many networks with many hosts per network, and Class C would be many networks with few hosts per network.