IP Address Classes:

Class A: 1.0.0.0 to 126.255.255.255
Class B: 128.0.0.0 to 191.255.255.255
Class C: 192.0.0.0 to 223.255.255.255
Class D/Multicast: 224.0.0.0 to 239.255.255.255

There can only be 126 Class A networks, but each network can have 16,777,214
hosts.
There can be 16,384 Class B networks, and each network can have 65,534 hosts.
There can be 2,697,152 Class C networks, although each network can only have
254 hosts.

A Class A address must have the first bit off, or 0.
A Class B address must have the first two bits set to 10.
A Class C address must have the first three bits set to 110.