2.7 Using IP Unnumbered
2.7.5 RIP with IP unnumbered configured improperly

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.