3.5. IP subnetting support

OSPF attaches an IP address mask to each advertised route. The mask indicates the range of addresses being described by the particular route. For example, a summary advertisement for the destination 128.185.0.0 with a mask of 0xffff0000 actually is describing a single route to the collection of destinations 128.185.0.0 - 128.185.255.255. Similarly, host routes are always advertised with a mask of 0xffffffff, indicating the presence of only a single destination.

Including the mask with each advertised destination enables the implementation of what is commonly referred to as variable- length subnetting. This means that a single IP class A, B, or C network number can be broken up into many subnets of various sizes. For example, the network 128.185.0.0 could be broken up into 62 variable-sized subnets: 15 subnets of size 4K, 15 subnets of size 256, and 32 subnets of size 8. Table 7 shows some of the resulting network addresses together with their masks:

                  Network address   IP address mask   Subnet size
                  _______________________________________________
                  128.185.16.0      0xfffff000        4K
                  128.185.1.0       0xffffff00        256
                  128.185.0.8       0xfffffff8        8

                         Table 7: Some sample subnet sizes.

There are many possible ways of dividing up a class A, B, and C network into variable sized subnets. The precise procedure for doing so is beyond the scope of this specification. This specification however establishes the following guideline: When an IP packet is forwarded, it is always forwarded to the network that is the best match for the packet's destination. Here best match is synonymous with the longest or most specific match. For example, the default route with destination of 0.0.0.0 and mask 0x00000000 is always a match for every IP destination. Yet it is always less specific than any other match. Subnet masks must be assigned so that the best match for any IP destination is unambiguous.

The OSPF area concept is modelled after an IP subnetted network. OSPF areas have been loosely defined to be a collection of networks. In actuality, an OSPF area is specified to be a list of address ranges (see Section C.2 for more details). Each address range is defined as an [address,mask] pair. Many separate networks may then be contained in a single address range, just as a subnetted network is composed of many separate subnets. Area border routers then summarize the area contents (for distribution to the backbone) by advertising a single route for each address range. The cost of the route is the minimum cost to any of the networks falling in the specified range.

For example, an IP subnetted network can be configured as a single OSPF area. In that case, the area would be defined as a single address range: a class A, B, or C network number along with its natural IP mask. Inside the area, any number of variable sized subnets could be defined. External to the area, a single route for the entire subnetted network would be distributed, hiding even the fact that the network is subnetted at all. The cost of this route is the minimum of the set of costs to the component subnets.