Discontiguous Subnets

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 Router B ignores this advertisement, because it can already get to network It is directly attached to network (The opposite is also true: Router B advertises that it can get to network, but Router A ignores this information.) This means that the routers cannot reach remote subnets of network

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 Router B advertises that it can get to networks (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 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.

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  • Giacobbe
    What hs discontiguous network?
    5 months ago

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