I suppose the key to VLSM is that not all routing
protocols support it. VLSM is not supported in RIP or
IGRP. You network design will be impacted by the
routing protocol you chose or have to support for
legacy reasons.
If you are stuck with RIP because IBM only support
RIPs or OSPF (in our enviroment anyway) then once you
have decided on the mask length it must stay that
length. If you make the mask 255.255.255.192 ie 64
addresses per subnet then every subnet you ever use is
length 64.RIP cannot handle different masks. If you
define a point-to-point serial link then you have to
allocated 64 address even though you only need two.
VLSM allows a subnet mask of 255.255.255.252 for
serial point-to-point links and then if you have a
building of 2000 PCs you can create a subnet of
255.255.240.0. If you didnt have VLSM supported in the
routing protocol then that building would have to have
about 30 subnets 64 address subnets defined and
default gateways etc to support all those IP
addresses. Or all you serials would use huge 2000 node
chunks of your IP allocation. (ie poor design )
hope this helps,
DM
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