Figure 19 illustrates how a planned and thoughtful allocation of VLSM

can reduce the size of an organization's routing tables. Notice how

Router D can summarize the six subnets behind it into a single adver 

tisement (11.1.253.0/24) and how Router B can aggregate all subnets

behind it into a single advertisement (11.1.0.0/16). Likewise, Router C

can summarize the six subnets behind it into a single advertisement

(11.253.0.0/16). Finally, since the subnet structure is not visible outside

of the organization, Router A injects a single route into the global Inter 

net's routing table 11.0.0.0/8 (or 11/8). 

VLSM Design Considerations

When developing a VLSM design, the network designer must recur 

sively 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 hier 

archy: 

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 on this level's largest subnet today?

4 How many hosts will be on this level's largest subnet be in the future?  

At each level, the design team must ensure that they have enough extra

bits to support the required number of subentities in the next levels of

recursion. 

Assume that a network is spread out over a number of sites. For exam 

ple, if an organization currently has three campuses, it probably needs 3

bits of subnetting (23 = 8) 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 work 

groups. Following this hierarchical model, the top level is determined

by the number of campuses, the middle level is based on the number of

buildings at each site, and the lowest level is determined by the maxi 

mum number of subnets and 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 bot 

tom level. At the bottom level, they must make sure that the leaf sub 

nets are large enough to support the required number of hosts. When

the addressing plan is deployed, the addresses from each site must be

aggregated into a single address block that keeps the backbone routing

tables from becoming too large. 

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