List of All Possible Valid Class A, B, and C Network Numbers

^[*] The Valid Network Numbers column shows actual network numbers. There are several reserved cases. For example, network 0.0.0.0 (originally defined for use as a broadcast address) and 127.0.0.0 (still available for use as the loopback address) are reserved. Networks 128.0.0.0, 191.255.0.0, 192.0.0.0, and 223.255.255.0 also are reserved.

You can see that Class A networks can begin with anything from 1 to 126 in the first octet. So, 1.0.0.0, 2.0.0.0, 3.0.0.0, 4.0.0.0, and so on, up to 126.0.0.0, are valid Class A network numbers. Similarly, Class B networks start with anything from 128 through 191. However, because a Class B network has a 2-octet network part, the network numbers are 128.1.0.0, 128.2.0.0, 128.3.0.0, and so on, up through 191.254.0.0. Finally, Class C networks' first octet ranges from 192 through 223, with the first 3 octets comprising the network part of the addresses in those networks.