|  | 10.5 | Reserved Address Space
| 10.5.1 | Purposes for network IDs and broadcast addresses

If your computer wanted to communicate with all of the devices on a network, it would be quite unmanageable to write out the IP address for each device. You might try two hyphenated addresses, indicating that you are referring to all devices within a range of numbers, but that, too, would be quite unmanageable. There is, however, a shorter method.

An IP address that ends with binary 0s in all host bits is reserved for the network address (sometimes called the wire address). Therefore, as a Class A network example, 113.0.0.0 is the IP address of the network containing the host 113.1.2.3. A router uses a network's IP address when it forwards data on the Internet. As a Class B network example, the IP address 176.10.0.0 is a network address.

The decimal numbers that fill the first two octets in a Class B network address are assigned and are network numbers. The last two octets contain 0s, because those 16 bits are for host numbers, and are used for devices that are attached to the network. The IP address in the example (176.10.0.0) is reserved for the network address. It will never be used as an address for any device that is attached to it.

If you wanted to send data to all of the devices on a network, you would need to use a broadcast address. A broadcast occurs when a source sends out data to all devices on a network. To ensure that all of the devices on the network pay attention to the broadcast, the sender must use a destination IP address that all of them can recognize and will pick up. Broadcast IP addresses end with binary 1s in the entire host part of the address (the host field).

For the network in the example (176.10.0.0) , where the last 16 bits make up the host field (or host part of the address), the broadcast that would be sent out to all devices on that network would include a destination address of 176.10.255.255 (since 255 is the decimal value of an octet containing 11111111).

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