Understanding VLSM

This section provides scenarios on using VLSM and indicates the issues it can cause. Remember that VLSM refers to using different masks on the same major net. In Figure 4-6, the major net of 168.71.0.0 has two different masks— 255.255.255.0 and 255.255.255.252.

Figure 4-6. 168.71.0.0 has two masks.

VLSM Using RIP and IGRP

In general, RIP V1 and IGRP do not support VLSM because they do not include the subnet masks associated with routes in their routing advertisements. This forces routers to derive the correct masks from the interfaces the routes are received over. Using different masks creates the mask ambiguity problem ...

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