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The way these addresses are used varies according to the class of the network, so all you can say with certainty is that the 32-bit IPv4 address is divided in some way to create an address for the network and an address for each host. In general, though, the higher-order bits of the address make up the network part of the address, and the rest constitutes the host part of the address. In addition, the host part of the address can be divided further to allow for a subnetwork address. For more detail on this addressing scheme, see the “IPv4 Address Classifications” and “Understanding Subnets” sections later in this chapter.

Some host addresses are reserved for special use. For example, in all network addresses, host numbers 0 and 255 are reserved. An IPv4 host address with all host bits set to 0 identifies the network itself; so refers to network 52. An IP address with all host bits set to 255 is known as a broadcast address. The broadcast address for network 204.176 is

A datagram sent to this address is automatically sent to every individual host on the 204.176 network.

ARIN (American Registry of Internet Numbers) assigns and regulates IP addresses on the Internet; you can get one directly from ARIN, or you can ask your Internet service provider (ISP) to secure an IP address on your behalf. Another strategy is to obtain your address from ARIN and only use it internally until you are ready to connect to the Internet.

If you are setting up an intranet and you don’t want to connect to the outside world through the Internet, you don’t need to register the IP addresses you use on your intranet with ARIN. Registering your addresses with ARIN simply ensures that the addresses you propose to use are unique over the entire Internet. If you are never going to connect to the Internet, there’s no reason to worry about whether those addresses are redundant with a computer that isn’t even on your network.

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