In all of the previous examples of subnetting, notice that the same subnet mask was applied for all the subnets. This means that each subnet has the same number of available host addresses. You can need this in some cases, but, in most cases, having the same subnet mask for all subnets ends up wasting address space. For example, in the Sample Exercise 2 section, a class C network was split into eight equal−size subnets; however, each subnet did not utilize all available host addresses, which results in wasted address space.
Figure 4 illustrates this wasted address space.
Figure 4
Figure 4 illustrates that of the subnets that are being used, NetA, NetC, and NetD have a lot of unused host address space. It is possible that this was a deliberate design accounting for future growth, but in many cases this is just wasted address space due to the fact that the same subnet mask is being used for all the subnets.Variable Length Subnet Masks (VLSM) allows you to use different masks for each subnet, thereby using address space efficiently.
