IPv4 addresses are divided into classes A, B and C. Class A provides addresses for a small number of very large networks. Class C provides addresses for a large number of small networks and class B provides a balance between the two.
IPv6 has 22 classes of address. Classes range in size from
of the address space to 
of the address
space. Of the 22 address classes 17 are reserved for future
expansion. The remaining 5 classes represent approximately 15% of the
address space (see the pie chart).
Figure 2: The IPv6 Address Space