A distributed system consists of many programs executing on a number of computers communicating with each other using some transmission medium. It is very difficult to test such systems, since programs would be running on a number of physically separated computers. The loopback address is used when distributed programs are brought together on the same computer and tested. It is an IP address which ensures that when it is used in a program communication occurs with a program running on the same computer. The loopback address is an IP address with a value 127.0.0.1 with the address expressed in dotted quad notation. A whole distributed system can be executed on the same computer connected through loopback addresses. The term is derived from the fact that when a message is sent to the loopback address it loops back to the original computer.