Internet Core Protocols: The Definitive Guide

Classless Inter-Domain Routing (CIDR)

All told, there are around 4.3 billion possible host addresses (there are fewer if you don’t consider Class D and E addresses, which cannot be used as host addresses). Unfortunately, the class-based structure of the addressing scheme used by IP placed heavy restrictions on the distribution of these addresses.

Every time a Class A address was assigned to an organization, almost 17 million host addresses went with it. If all 126 of the Class A networks were assigned, then two billion of the possible addresses were gone. If all of the available Class B networks were assigned, then another billion host addresses were gone as well.

Class C addresses represented the biggest problem, however, for two reasons. First, there are fewer IP addresses available in all of the Class C networks than there are with the other classes (only about 600 million possible node addresses are available from all of the Class C networks combined). Second, Class C networks were the most popular, since they reflected the size of the majority of the LANs in use.

However, every time a Class C address was assigned, 256 possible host addresses went with it. Organizations who had three segments but who only had 60 devices were wasting over 700 possible addresses (3 segments × 254 addresses = 762 addresses - 60 active nodes = 702 inactive addresses). Whether or not all of the addresses were actually put to use or not is irrelevant because they were assigned to a specific network ...