You can use a local loopback address to test your local TCP/IP host without accessing the network. For the AF_INET family, the class A network address 127.0.0.0 is the default loopback address. For AF_INET6, the network address ::1 is the default loopback address. Depending on the address family, you can specify 127.0.0.0 (AF_INET) or ::1 (AF_INET6). Additional loopback addresses can be configured by your TCP⁄IP administrator.

You can use the loopback address with any TCP⁄IP command that accepts IP addresses, although you might find it particularly useful in conjunction with FTP and PING commands. When you issue a command with a loopback address, the command is sent from your local host client to the local TCP/IP host where it is recognized as a loopback address and is sent to your local host server.

Using a loopback address on commands allows you to test client and server functions on the same host for proper operation.

The information you receive reflects the state of your system and tests the client and server code for proper operation. If the client or server code is not operating properly, a command message is returned.