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This
scenario shows what happens when the rule is violated that requires the
same mask to be used on all occurrences of the same major network's
subnets. In figure to the left,
RouterC's ethernet 0 interface now has a mask of 255.255.255.240. This is
an example of variable length subnet masking (VLSM). In this case, major
network 168.71.0.0 now has two masks: 255.255.255.0 and 255.255.255.240.
In the output of the
show interface
commands from RouterC,
you can see the new mask in use. (see
RouterC#show interface ethernet 0
and
RouterC#show interface serial 1
command outputs ). Note that no reference to the mask used
on Ethernet 0 is in the output from serial 1. You must always check the
interface that the address is being derived from to determine which subnet
mask is in use.
Note: When you are troubleshooting IP unnumbered problems always
check the show interface command from the interface that is providing the
address.
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