Before the advent of classless addressing, one could tell the size of a network by the first few bits of an IP address. If the first bit was set to zero (the first byte being from 0 to 127), the address was a class A network. Values from 128 to 191 were used for class B networks whereas values between 192 and 223 were used for class C networks. Class D, with values from 224 to 239 (the first three bits set to one and the fourth to zero), was reserved for IP multicast. Source: STREBE, Matthew and PERKINS, Charles, Firewalls 24seven, Sybex 2000, Chapter 3: TCP/IP from a Security Viewpoint.