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TCP/IP Guide by Charles M. Kozierok

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IP Subnet Masks, Notation, and Subnet Calculations

Subnetting divides an organization's network into a two-level structure of subnets and hosts that is entirely internal and hidden from all other organizations on the Internet. One of the many advantages of this is that each organization gets to make its own choice about how to divide the classful host ID into subnet ID and host ID.

In a nonsubnetted classful environment, routers use the first octet of the IP address to determine what the class of the address is, and from this they know which bits are the network ID and which are the host ID. When you use subnetting, these routers also need to know how that host ID is divided into subnet ID and host ID. However, this division can be arbitrary for ...

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