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Variable Length Subnet Masks (VLSMs)
To take one network and create many networks using subnet
masks of different lengths on different types of network designs. This is called VLSM networking.
Neither RIPv1 nor IGRP routing protocols have a field for subnet information, so the subnet
information gets dropped. What this means is that if a router running RIP has a subnet mask of
a certain value, it assumes that all interfaces within the classful address space have the same subnet
mask. This is called classful routing, and RIP and IGRP are both considered classful routing
protocols. (I’ll be talking more about RIP and IGRP in Chapter 6, “IP Routing.”) If you mix and
match subnet mask lengths in a network running RIP or IGRP, that network just won’t work.
Classless routing protocols, however, do support the advertisement of subnet information.
Therefore, you can use VLSM with routing protocols such as RIPv2, EIGRP, and OSPF. The
benefit of this type of network is that you save a bunch of IP address space with it.
As the name suggests, with VLSMs we can have different subnet masks for different router
interfaces.
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