Notice that first few bits in each class have a fixed value. For example a class A address should have the first bit set to 0. Similarly class C should have first 2 bits set to 1 and the third bit set to 0. Another point to note is that though the class A range is from 0 to 127, the address 0.0.0.0 is reserved to mean “any network” and 127.0.0.1 is reserved as a loopback address which refers to the host itself. So the class A network is restricted to the 1-126 range