Subnet Design Considerations
The deployment of an addressing plan requires careful thought. Four
key questions that must be answered before any design should be
undertaken are:
1 How many total subnets does the organization need today?
2 How many total subnets will the organization need in the future?
3 How many hosts are on the organization's largest subnet today?
4 How many hosts will there be on the organization's largest subnet in
the future?
The first step in the planning process is to take the maximum number of
subnets required and round up to the nearest power of two. For exam 
ple, if an organization needs nine subnets, 23 (or 8) will not provide
enough subnet addressing space, so the network administrator will
need to round up to 24 (or 16). 
The network administrator must always allow adequate room for
growth. For example, although 14 subnets are required today, 16 sub 
nets might not be enough in two years when the 17th subnet needs to
be deployed. In this case, it would be wise to select 25 (or 32) as the
maximum number of subnets. 
The second step is to ensure that there are enough host addresses for
the organization's largest subnet. If the largest subnet needs to support
50 host addresses today, 25 (or 32) will not provide enough host address
space so the network administrator will need to round up to 26 (or 64). 
The final step is to make sure that the organization's address allocation
provides enough bits to deploy the required subnet addressing plan.
For example, if the organization has a single /16, it could easily deploy 4
bits for the subnet number and 6 bits for the host number. However, if
the organization has several /24s and it needs to deploy nine subnets, it
may have to subnet each of its /24s into four subnets (using 2 bits) and
then build the network by combining the subnets of three /24 network
An alternative solution would be to deploy network numbers from the
private address space (RFC 1918) for internal connectivity and use a
Network Address Translator (NAT) to provide external Internet access. 
Subnet Example #1
An organization is assigned the network number and it
needs to define six subnets. The largest subnet is required to support 25
1 1






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