Efficient Address Allocation

How does all of this lead to the efficient allocation of the IPv4 address space? In a classful environment, an Internet Service Provider (ISP) can only allocate /8, /16, or /24 addresses. In a CIDR environment, the ISP can carve out a block of its registered address space that specifically meets the needs of each client, provides additional room for growth, and does not waste a scarce resource.

Assume that an ISP has been assigned the address block 206.0.64.0/18 . This block represents 16,384 (2 14 ) IP addresses which can be interpreted as 64 /24s. If a client requires 800 host addresses, rather than assigning a Class B (and wasting 64,700 addresses) or four individual Class Cs (and introducing 4 new routes into the global Internet routing tables), the ISP could assign the client the address block 206.0.68.0/22, a block of 1,024 (2 10 ) IP addresses (4 contiguous /24s). The efficiency of this allocation is illustrated in Figure 28.