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

numbers. 

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

Given

An organization is assigned the network number 193.1.1.0/24 and it

needs to define six subnets. The largest subnet is required to support 25

hosts. 

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