VLSM Design Considerations
When developing a VLSM design, the network designer must recursively ask the same set of questions as for a traditional subnet design. The same set of design decisions must be made at each level of the hierarchy:
1) How many total subnets does this level need today?
2) How many total subnets will this level need in the future?
3) How many hosts are there on this level’s largest subnet today?
4) How many hosts will there be on this level’s largest subnet be in the future?
At each level, the design team must make sure that they have enough extra bits to support the required number of sub-entities in the next and further levels of recursion.
Assume that a network is spread out over a number of sites. For example, if an organization has three campuses today it probably needs 3-bits of subnetting (2 3 = 
to allow the addition of more campuses in the future. Now, within each campus, there is likely to be a secondary level of subnetting to identify each building. Finally, within each building, a third level of subnetting might identify each of the individual workgroups. Following this hierarchical model, the top level is determined by the number of campuses, the mid-level is based on the number of buildings at each site, and the lowest level is determined by the “maximum number of subnets/maximum number of users per subnet” in each building.
The deployment of a hierarchical subnetting scheme requires careful planning. It is essential that the network designers recursively work their way down through their addressing plan until they get to the bottom level. At the bottom level, they must make sure that the leaf subnets are large enough to support the required number of hosts. When the addressing plan is deployed, the addresses from each site will be aggregable into a single address block that keeps the backbone routing tables from becoming too large.
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