Subnets

An IP address must identify both the host and the network where that host resides. As you learned in Hour 4, “The Internet Layer,” the IP address class system gives a clue for how to distinguish the network and host portion of the address. But the address class system is too inflexible to do the job alone. In the real world, networks come in all sizes, and many networks are divided into smaller units. Furthermore, the world is running out of class-level networks. ISPs and network admins need a flexible means for subdividing a class-level network so that datagrams arrive at routers serving a smaller address space.

Subnetting lets you break the network into smaller units called subnets. The concept of a subnet originally evolved around ...

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