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TCP/IP Guide by Charles M. Kozierok

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IP Custom Subnet Masks

A default subnet mask doesn't really represent subnetting because you are assigning zero bits to the subnet ID. To do real subnetting, you must dedicate at least one of the bits of the presubnetted host ID to the subnet ID.

Since you can choose the dividing point between subnet ID and host ID to suit the network, this is sometimes called customized subnetting. The subnet mask that you use when creating a customized subnet is, in turn, called a custom subnet mask. The custom subnet mask is used by network hardware to determine how you have decided to divide the subnet ID from the host ID in the network.

Deciding How Many Subnet Bits to Use

The key decision in customized subnetting is how many bits to take from the host ID portion ...

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