MCSE : Security
6.
How to maximize the number of hosts for a given number of subnets:
Determining the subnet mask that allows maximum
number of hosts:
Let us consider an IP address 196.202.56.0 with
four subnets and maximize the number of host for the given subnets.
The steps involved are as below:
I.
The number of subnets required are four. We need to add
subnets of all ones and all zeros to this. This is because all zeros
and all ones subnets belong to "this subnet" and "all subnets"
broadcasts and can not be used. Therefore, the total number of
subnets to be reserved is 4+2 = 6.
II. We want to implement maximum possible Hosts. Therefore, we need
to minimize the number of subnets. This minimum number is 6 here. If
we reserve 2 bits, it results in only 2^2=4 subnets which is
less than 6. Therefore, we have to reserve 3 bits for
implementing subnets, resulting in 2^3=8 subnets. This is now
optimized for maximum number of Hosts (as we have optimized for
minimum number of subnets).
III. Write the 3 bits available for subnetting in fourth octet in
the form 11100000 (Five 0s being Host bits). The decimal equivalent
is 2^7+2^6+2^5
=
128 + 64 +32 = 224.
IV. Therefore, the subnet mask required is 255.255.255.224.
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