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Reserved IP addresses

Reserved IP addresses
9.2.5 This page will describe the types of reserved IP addresses.


Certain host addresses are reserved and cannot be assigned to devices on a network. These reserved host addresses include the following:

• Network address – Used to identify the network itself

In Figure , the section that is identified by the upper box represents the 198.150.11.0 network. Data that is sent to any host on that network (198.150.11.1- 198.150.11.254) will be seen outside of the local area network as 198.159.11.0. The only time that the host numbers matter is when the data is on the local area network. The LAN that is contained in the lower box is treated the same as the upper LAN, except that its network number is 198.150.12.0.

• Broadcast address – Used for broadcasting packets to all the devices on a network

In Figure , the section that is identified by the upper box represents the 198.150.11.255 broadcast address. Data that is sent to the broadcast address will be read by all hosts on that network (198.150.11.1- 198.150.11.254). The LAN that is contained in the lower box is treated the same as the upper LAN, except that its broadcast address is 198.150.12.255.

An IP address that has binary 0s in all host bit positions is reserved for the network address. In a Class A network example, 113.0.0.0 is the IP address of the network, known as the network ID, containing the host 113.1.2.3. A router uses the network IP address when it forwards data on the Internet. In a Class B network example, the address 176.10.0.0 is a network address, as shown in Figure .

In a Class B network address, the first two octets are designated as the network portion. The last two octets contain 0s because those 16 bits are for host numbers and are used to identify devices that are attached to the network. The IP address, 176.10.0.0, is an example of a network address. This address is never assigned as a host address. A host address for a device on the 176.10.0.0 network might be 176.10.16.1. In this example, “176.10” is the network portion and “16.1” is the host portion.

To send data to all the devices on a network, a broadcast address is needed. A broadcast occurs when a source sends data to all devices on a network. To ensure that all the other devices on the network process the broadcast, the sender must use a destination IP address that they can recognize and process. Broadcast IP addresses end with binary 1s in the entire host part of the address.

In the network example, 176.10.0.0, 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. This is because 255 is the decimal value of an octet containing 11111111.

The next page will discuss public and private IP addresses.

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