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Limits to the Classful Addressing System

Not all organizations’ addressing requirements fit well into one of these three classes. Classful allocation of address space often wasted many addresses, which exhausted the availability of IPv4 addresses. For example, a company that had a network with 260 hosts would need to be given a class B address with more than 65,000 addresses.

Even though this classful system was all but abandoned in the late 1990s, the remnants of it are in effect in networks today. For example, when you assign an IPv4 address to a comput-er, the operating system examines the address being assigned to determine whether it is a class A, class B, or class C address. The operating system then assumes the prefix used by that class and makes the appropriate subnet mask assignment.

Another example is the assumption of the mask by some routing protocols. When some routing protocols receive an advertised route, it can assume the prefix length based on the class of the address.

Classless Addressing

The system that is currently in use is referred to as classless addressing. With the classless system, address blocks appropriate to the number of hosts are assigned to companies or organizations without regard to the unicast class. Associated with this classless addressing system are other practices, such as using networks of fixed sized, that have made IPv4 addressing more viable.

Assigning Addresses

Network personnel must design the addresses scheme for the network. The following sec-tions present the practices and processes of providing addresses for networks.

Planning to Address the Network

The allocation of network layer address space within the corporate network needs to be well designed. Network administrators should not randomly select the addresses used in the networks.

Network administrators should plan and document the allocation of these addresses inside the networks for the following purposes:

■   Preventing duplication of addresses

■   Providing and controlling access

■   Monitoring security and performance