Which class provides the largest number of networks in classful IPv4?

• A. Class C
• B. Class A
• C. Class B
• D. Class D

Answer: (A)

In classful IPv4, how many networks you can have depends on how many bits are used for the network portion of the address. Class A fixes one bit (the first) and leaves 7 bits for the network, giving 2^7 = 128 possible networks. Class B fixes two leading bits, leaving 14 bits for the network, giving 2^14 = 16,384 networks. Class C fixes three leading bits, leaving 21 bits for the network, giving 2^21 = 2,097,152 networks. Since Class C uses the most bits for the network portion, it yields the largest number of networks. (Class D is for multicast and isn’t organized into networks in the same way.)