6.2 Network: Second Period of History

In 1993, the RFC 1517 1520 Classless Inter-Domain Routing (CIDR) specifications were released. These specifications are only seldom referred to nowadays, but had changed the understanding of the term ‘network’ in the context of the Internet. Networks started to be regarded in terms of network masks instead of network classes.

The computer address part of the IP address was divided into two parts, the subnetwork address and the computer address.

Figure 6.2: IP address structure

From the network address point of view, the network address and the subnetwork address form one unit. Briefly speaking, the IP address part with ones only in the network mask specifies the network. At this point, terminology becomes ambiguous. In some cases, the term network applies to an A, B, or C Class network while in other cases the term network refers to that part of the IP address for which there are ones in the corresponding network mask. If we forget the...