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Windows NT TCP/IP Network Administration by Robert Bruce Thompson, Craig Hunt

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Subnets

The structure of an IP address can be locally modified by using host address bits as additional network address bits. Essentially, the dividing line between network address bits and host address bits is moved, creating additional networks, but reducing the maximum number of hosts that can belong to each network. These newly designated network bits define a network within the larger network, called a subnet.

Organizations usually decide to subnet in order to overcome topological or organizational problems. Subnetting allows decentralized management of host addressing. With the standard addressing scheme, a central administrator is responsible for managing host addresses for the entire network. By subnetting, the administrator can delegate address assignment to smaller organizations within the overall organization—which may be a political expedient, if not a technical requirement. If you don’t want to deal with the data processing department, assign them their own subnet and let them manage it themselves.

Subnetting can also be used to overcome hardware differences and distance limitations. IP routers can link dissimilar physical networks together, but only if each physical network has its own unique network address. Subnetting divides a single network address into many unique subnet addresses, so that each physical network can have its own unique address.

A subnet is defined by changing the bit mask of the IP address. A subnet mask functions in the same way as a normal address ...

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