Subnetting is a more general approach of how to break up major networks into smaller parts. VLSM is a subset of subnetting and is a particular approach to creating subnets where subnets within the same major network can use different subnet masks.
The main alternative to VLSM is Fixed Length Subnet Masking. This was the earlier approach to subnetting and its primary characteristic is that within a major network all subnets must use the same length subnet mask.
HTH
Rick
HTH
Rick
