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IP Subnetting Step #2: The Key Design Trade-off: Partitioning Network Address Host Bits
(Page 1 of 3)
After we complete our brief requirements
analysis, we should know the two critical
parameters that we must have in order to subnet our network: the number
of subnets required for the network, and the maximum number of hosts
per subnetwork. In using these figures to design our subnetted network,
we will be faced with the key design decision in subnetting: how to
divide the 8, 16 or 24 bits in the classful host ID into
subnet ID and host ID.
Deciding How Many Bits to Use for the Subnet ID and Host ID
Put another way, we need to decide
how many bits to steal from the host ID to use for the subnet
ID. As I introduced in the
topic on custom subnet masks, the fundamental
trade-off in choosing this number is as follows:
- Each bit taken from the host ID for the subnet
ID doubles the number of subnets that are possible in the network.
- Each bit taken from the host ID for the subnet
ID (approximately) halves the number of hosts that are possible within
each subnet on the network.
There are six possible ways this
decision can be made for a Class C network, as illustrated in Figure 73.
Figure 73: Subnetting Design Trade-Off For Class C Networks
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The relationship between
the bits and the number of subnets and hosts is as follows:
- The number of subnets allowed in the network
is two to the power of the number of subnet ID bits.
- The number of hosts allowed per subnet is two
to the power of the number of host ID bits, less two.
We subtract two from the number of
hosts in each subnet to exclude the special
meaning cases where the host ID
is all zeroes or all ones. As I explained in the
topic on custom subnetting, this exclusion
was originally also applied to the subnet ID, but is no longer in newer
systems.
Now, to choose how many bits to use
for the subnet we could use trial and error. By this I mean we
could try to first calculate the number of subnets and hosts when we
use one bit for the subnet ID and leave the rest for the host ID. We
could then try with two bits for the subnet ID, and then try with three
and so on. This would be silly, however; it's time consuming and makes
it hard for us to choose the best option. There's an easier method:
we can use the subnetting
summary tables. They let us look at all
our options and usually see immediately the best one for us.
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Version 3.0 - Version Date: September 20, 2005
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