As you've seen, certain IP addresses and address ranges have special uses:

On classful networks, all the bits in the network ID cannot be set to 0 because this expression is reserved to indicate a host on a local network. Similarly, on a classful network all the bits in the host ID cannot be set to 0 because this is reserved to indicate the IP network number.

Table 24-1 lists the ranges of network numbers based on address classes. You cannot assign the network number to a network interface. The network number is common for all network interfaces attached to the same logical network. On a nonclassful network, the network number is the first IP address in the range of IP addresses for the associated subnet—as specified in RFC 1812.

When you apply all the rules for IP addresses, you find that many IP addresses cannot be used by hosts on a network. This means the first available host ID and last available host ID are different from the range of available IP addresses. Table 24-2 shows how these rules apply to classful networks. On a nonclassful network, the same rules apply—you lose the first and last available host ID from the range of available IP addresses.

Inside Out: Routers, gateways, and bridges connect networks

A router is needed for hosts on a network to communicate with hosts on other networks. It is standard convention for the network router to be assigned the first available host ID. On Windows systems, you identify the address for the router as the gateway IP address for the network. Although the terms "gateways" and "routers" are often used interchangeably, the two technically are different. A router is a device that sends packets between network segments. A gateway is a device that performs the necessary translation so that communication between networks with different architectures is possible, such as between Novell NetWare and Windows networks. When working with networks, you might also hear the term "bridge." A bridge is a device that directs traffic between two network segments using physical machine addresses (Media Access Control, or MAC, addresses). Routers, gateways, and bridges can be implemented in hardware as separate devices or in software so that a system on the network can handle the role as a network router, gateway, or bridge as necessary.