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Restriction: Fixed-length subnet masks Connected: An Internet Encyclopedia
Restriction: Fixed-length subnet masks

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Restriction: Fixed-length subnet masks Because the older, classful routing protocols don't convey prefix lengths, routers using these protocols have no way of knowing the prefix lengths of incoming routes! Therefore, they have no choice but to assume that subnet routes from different parts of their classful network all use the same subnet mask that they use. This leads to a further restriction on subnet design - the same subnet mask must be used throughout the classful network, a practice called Fixed Length Subnet Masking (FLSM), as opposed to Variable Length Subnet Masking (VLSM), supported by the newer, classless breed of routing protocols.

For example, assume that we've assigned a subnet. If we're using RIP (an older, classful, FLSM routing protocol), we can not now assign as another subnet. Why? Because both subnets are in the same class C network (, yet they have different prefix lengths. FLSM forbids this. We could assign, however, since both subnets in the network would share the same prefix length of 26.

Next: Restriction: Contiguous Subnets

Connected: An Internet Encyclopedia
Restriction: Fixed-length subnet masks


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