When you want to subnet a classful network, you need to take inventory of what you know and what you need to know. The following steps may be useful.
1. What do I know?
You must know either the number of subnets you want or the number of hosts that you want per subnet.
What do you know? Number of Subnets: _________ or
Number of Hosts: _________
2. Now that you know what is required, you must determine the number of bits required to produce the required result. To do this, use 2n-2> = number that your need (hosts or subnets). For example, if you needed to subnet a network with six subnets, you would need 2n-2 >= 6. When you solve this formula, you find that n=3. This will yield 6 useable networks.
3. With the number of bits required to subnet now known, write the network you are subnetting along with the classful subnet mask in binary.
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4. After you have this written , add the subnet bits to the right of the classful subnet mask. Replicate the table you produced above and add the number of ones to that you calculated above to the classful subnet mask. The ones that you add represent the subnet.
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5. Subnetting rules are few, but must be applied.
The network address nor the subnet address may not be all 0’s of all 1’s.
The address that has all 0’s in the host positions (0’s in the subnet mask) is the network or subnet address.
The address that has all 1’s in the host position (0’s in the subnet mask) is the broadcast address.
The address that has all 1’s in the subnet ID is called the broadcast subnet and is not useable.
The address that has all 0’s in the subnet ID is called the zero subnet and is not useable.
Now look at an example at this link Subnet Example
