IPv6, 2nd Edition

Chapter 2 • What’s Wrong with IPv4? 29

the calculation of address density, reaching the conclusion that the IPv4

address space continues to draw near to practical exhaustion. Both of these

documents provide excellent insight into the growth of the Internet and

the difficulty of efficiently using the IPv4 address space.

Class A networks accounted for half of the entire IPv4 address space; Class

B networks accounted for 25% and Class C networks for 12.5%. A full one-

eighth of the address space was reserved for future use (with half of that

later allocated for IP multicast addresses), right off the top. Most firms of

any size at all wanted Class B networks, ...