A loopback test is a loop test of transmission channels, especially duplex channels. A test pattern is sent from the transmitting station to the opposite side and sent back from this receiving station to the transmitting station. The returned test pattern is then compared with the original pattern. This test can be used to determine signal changes in the form of noise, distortion, signal loss or bit errors.
In the TCP/ IP environment, the loopback test is performed using a special address, the loopback address. The IP address 127.0.0.1 is reserved for loopback tests, or for Classless Interdomain Routing( CIDR) the address 127.0.0.0/8. Most systems have assigned the IP address 127.0.0.1 to their interfaces, and for IPv6 the address ::1.
The connections are established via the Telnet protocol or the File Transfer Protocol( FTP), which are answered on the receiving side. If this test does not work, you can assume with high probability that the problem is to be found in your own computer. If the local loopback test works and still no connection can be established via the network, the errors are to be looked for on the other computer, the network or on the internal communication controller.
The loopback test is similar to the ping-pong procedure, where a message is sent to a host on the Internet and the host responds. The response ensures that the host is reachable. The response time is the ping time.