Reserved Addresses

Even with Classless IP, not every IP address is usable. Addresses with a first byte greater than 223 are reserved for Multi-cast and other special applications. There are also two Class A networks that are reserved for special purposes. The network address 0.0.0.0 designates a default gateway. This is used in routing tables to represent "All Other Network Addresses". Another special Class A network is 127.0.0.0, the loopback address. This network address is used to simplify programming, testing, and troubleshooting. It allows applications to communicate with the local host in the same manner as communicating with a remote host, without the need to look up the local address. There are other addresses that are used for special purposes. In every network the host address which is all Zeros identifies the network itself. This is called the Network Address and is used in routing tables to refer to the whole network. A host address which is all Ones is called the Broadcast Address or Announce Address for that network. For example, on the Class C network 205.217.146.0, the address 205.217.146.255 is the broadcast address. Packets addressed to the broadcast address will be received by all hosts on that network. The network address and the broadcast address should not be used as actual host addresses.