Under certain circumstances, it may be desirable to support subnets of a particular network being interconnected only through a path that is not part of the subnetted network. This is known as discontiguous subnetwork support.
Routers MUST support discontiguous subnetworks.
In classical IP networks, this was very difficult to achieve; in CIDR networks, it is a natural by-product. Therefore, a router SHOULD NOT make assumptions about subnet architecture, but SHOULD treat each route as a generalized network prefix.
The Internet has been growing at a tremendous rate of late. This has been placing severe strains on the IP addressing technology. A major factor in this strain is the strict IP Address class boundaries. These make it difficult to efficiently size network prefixes to their networks and aggregate several network prefixes into a single route advertisement. By eliminating the strict class boundaries of the IP address and treating each route as a generalized network prefix, these strains may be greatly reduced.
The technology for currently doing this is Classless Inter Domain Routing (CIDR) [INTERNET:15].
For similar reasons, an address block associated with a given network prefix could be subdivided into subblocks of different sizes, so that the network prefixes associated with the subblocks would have different length. For example, within a block whose network prefix is 8 bits long, one subblock may have a 16 bit network prefix, another may have an 18 bit network prefix, and a third a 14 bit network prefix.
Routers MUST support variable length network prefixes in both their interface configurations and their routing databases.