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IP Subnetting Step #1: Requirements Analysis
(Page 2 of 2)
Assessing Future Needs During Requirements Analysis
We need to analyze the requirements
above not only for the present network, but for the near future
as well. The current values for these two numbers represent how the
network needs to be designed today. However, designing only for the
present is not a good idea.
Suppose we have exactly four subnetworks
in our network now. In theory, we could use only two bits for the subnet
ID, since 22 is 4. However, if our company is growing rapidly,
this would be a poor choice. When we need to add a fifth subnet we'd
have a problem!
Similarly, consider the growth in
the number of hosts in a subnet. If the current largest subnet has 60
hosts, you don't want 6a bits for the host ID, because that limits you
to 62 hosts. You can divide large subnets into smaller ones, but this
may just mean unnecessarily additional work.
So, what is the “near future”?
The term is necessarily vague, because it depends on how far into the
future the organization wants to look. On the one hand, planning for
several years' growth can make sense, if you have enough IP addresses
to do it. On the other, you don't want to plan too far out, since changes
in the short term may cause you to completely redesign your network
anyway.
 Key Concept: To successfully subnet a network, you must begin by learning what the requirements of the network will be. The most important parameters to determine are the number of subnets required and the maximum number of hosts needed per subnet. Numbers should be based not just on present needs but requirements in the near future.
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