As mentioned earlier, classful routing protocols automatically summarize subnets. One side-effect of this is that discontiguous subnets are not supported. Subnets must be next to each other—that is, contiguous. Figure 6-3 shows an enterprise network with discontiguous subnets.
Figure 6-3. A Network with Discontiguous Subnets
With a classful routing protocol such as RIP version 1 or IGRP, Router A in Figure 6-3 advertises that it can get to network 10.0.0.0. Router B ignores this advertisement, because it can already get to network 10.0.0.0. It is directly attached to network 10.0.0.0. (The opposite is also true: Router B advertises that it can get to network 10.0.0.0, but Router A ignores this information.) This means that the routers cannot reach remote subnets of network 10.0.0.0.
To solve this problem, a classless routing protocol can be used. With a classless routing protocol, Router A advertises that it can get to networks 10.108.16.0/20. Router B advertises that it can get to networks 10.108.32.0/20. (To understand why the prefix length is 20, convert the network numbers to binary.) Because classless routing protocols understand prefixes of any length (not just 8, 16, or 24), the routers in Figure 6-3 can route to discontiguous subnets, assuming they are running a classless routing protocol, such as OSPF or EIGRP.
To configure the devices in the previous example with an old-style subnet mask rather than a prefix length, use a mask of 255.255.240.0. The first 4 bits of the third octet are set to 1s. A trick for determining the value of the relevant octet in a subnet mask is to subtract the number of summarized subnets from 256. In this example, there are 16 summarized subnets, so the relevant octet is 256 minus 16, or 240.
Was this article helpful?
What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.