Reserved IP Addresses
The following IP addresses are reserved and cannot be assigned to individual devices on a network:
- Network address: An IP address that has binary 0s in all host bit positions is reserved for the network address. The network address is used to identify the network itself. For example, 172.16.0.0 is an example of a Class B network address. In this address, the first two octets are reserved for the network address; that address is never used as an address for any device that is attached to it. 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. An example of an IP address for a device on the 172.16.0.0 network would be 172.16.16.1. Routers use network addresses when they search their IP route tables for destination network locations.
- Directed broadcast address: To send data to all the devices on a network, a broadcast address is used. Broadcast IP addresses end with binary 1s in the entire host part of the address (the host field). Consider the network in the example 172.16.0.0, in which 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 172.16.255.255. The directed broadcast is capable of being routed. However, for some versions of Cisco IOS Software, routing directed broadcasts is not the default behavior.
- Local broadcast address: If an IP device wants to communicate with all devices on the local network, it sets the destination address to all 1s (255.255.255.255) and transmits the packet. For example, hosts that do not know their network number and are asking some server for the number can use this address. The local broadcast is never routed.
- Local loopback address: A local loopback address is used to let the system send a message to itself for testing. A typical local loopback IP address is 127.0.0.1.
- Autoconfiguration IP addresses: Sometimes, neither a statically nor a dynamically configured IP address is found on startup. In such instances, hosts supporting IPv4 link-local addresses (RFC 3927) generate an address in the 169.254/16 prefix range. This address can be used only for local network connectivity and operates with many caveats, one of which is that it will not be routed. You will mostly see this address as a failure condition when a PC fails to obtain an address via DHCP.