CCENT/CCNA ICND1 100-101 Official Cert Guide

Finding VLSM Overlaps

Regardless of whether a design uses VLSM, the subnets used in any IP internetwork design should not overlap their address ranges. When subnets in different locations overlap their addresses, a router’s routing table entries overlap. As a result, hosts in different locations can be assigned the same IP address. Routers clearly cannot route packets correctly in these cases. In short, a design that uses overlapping subnets is considered to be an incorrect design and should not be used.

Note

Although I’ve not seen the term used in other places, just to have a term to contrast with VLSM, this book refers to the non-use of VLSM—in other words, using a single mask throughout a classful network—as static-length subnet masks (SLSM). ...