There are 4 classes of IPv4 networks:

A Class A network can have 16,777,214 computers on it. It has an IP address that starts with a number in the range 0 to 126. 127 is reserved for the local loopback. (Only 127.0.0.1 is ever used although this may change in the future).

A Class B network can have 65,534 computers on it. It has an IP address that starts in the range of 128 to 191.