A subnet is a network that falls within a Class A, B, or C network. Subnets are created by using one or more of the Class A, B, or C host bits to extend the network ID.
Subnet Masks
In order for subnetting to work, the router must be told which portion of the host ID should be used for the subnet network ID. It is accomplished by using another 32-bit number, known as a subnet mask.
Those IP address bits that represent the network ID are represented by a 1 in the mask, and those bits that represent the host ID appear as a 0 in the mask. As a result, a subnet mask always has a consecutive string of ones on the left, followed by a string of zeros.
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