How to Use Subnetting Charts
Creating subnetworks is a networking technique used to divide private IP address space within an organization or a Network Address Translation (NAT) activated network of an arbitrary size. Restricting networks to as few available addresses as possible is crucial to effective routing that wastes as little bandwidth as possible. Even experienced network administrators sometimes consult subnetting charts (or calculators) in order to take some of the guesswork out of planning networks.
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Instructions
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Determine how many hosts you need in a network before determining how many networks you need. The number of hosts you need automatically determines what class of private network you need. On the subnet chart, this is the furthest right column.
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Choose the appropriate class, A, B or C, then use that to determine how many subnets you need. Typically, this corresponds to the class, since unused subnets can be ignored. When creating classful subnets, in any case, the subnets will always be of equal size.
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Calculate the network addresses that comprise each subnet by combining the class with the number of addresses per network. For example, in a class C network of 8 subnets with 30 hosts, beginning at 192.168.0.0, the subnets are .0, .32, .64, .96, etc., with the first and last addresses being reserved for network and broadcast addresses, respectively.
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Use the right-most column of the subnetting chart to figure out the subnet mask, the part of an IP address that separates the network bit from the host bit, corresponding to the number of hosts, subnets and class your of subnetworks.
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