Reserved IP Addresses

Reserved IP Addresses

A component of an IP address with a value all bits 0 or all bits 1 has a special meaning:

• All bits 0: An address with all bits 0 in the host number portion is interpreted as this host (IP address with host address =0).

All bits zero in the network number portion is this network (IP address with network address =0).

When a host wants to communicate over a network, but does not yet know the network IP address, it may send packets with network address=0.

Other hosts on the network interpret the address as meaning this network. Their replies contain the fully qualified network address, which the sender records for future use.

All bits 1: An address with all bits 1 is interpreted as all networks or all hosts. For example, the following means all hosts on network 128.2 (class B address):

128.2.255.255

This is called a directed broadcast address because it contains both a valid network address and a broadcast host address.

• Loopback: The class A network 127.0.0.0 is defined as the loopback network. Addresses from that network are assigned to interfaces that process data within the local system. These loopback interfaces do not access a physical network.