Traditionally, the structure of an IP address differs depending on
the class of the network to which the host is
connected[1].
Class A Network
the first (or leftmost) field, or byte, of the address
specifies the network number and class, and can take values from 1
to 126.
Class B network
the leftmost two fields (bytes) specify the network number and
class. The first field is in the range from 128 to 191, and the
second from 1 to 254.
Class C network
the leftmost three fields specify network number and class. The
first field can be from 192 to 223, the second from 0 to 255, and
the third from 1 to 254.
[1] Since the
mid-1990s, this scheme has been replaced with the newer CIDR allocation algorithm. However, it's still
important to have a handle on the older system, because CIDR is
really an extension, and generalisation, of it.