Loopback Address

IP defines a loopback address used to test network applications.Programmers often use loopback testing for preliminary debugging after a network application has been created.To perform a loopback test,a programmer must have two application programs that are intended to communicate across a network.Each application includes the code needed to interact with TCP/IP protocol software.Instead of executing each program on a separate computer,the programmer runs both program on a single computer and instructs them to use a loopback IP address when communicating.

When one application sends data to another,data travels from the protocol stack to the IP software,which forwards it back up through the protocol stack to the second program.Thus,the programmer can test the program logic quickly without needing two computers and without sending packets across a network.IP reserves the network prefix 127/8 for use with loopback.The host address used with 127 is irrelevant that means all host addresses are treated the same.By convention.programmers often use host number 1,making 127.0.0.1 the most popular form of loopback.During loopback testing no packets ever leave a computer that means the IP software forwards packets from one application program to another.Consequently,the loopback address never appears in packet traveling across a network.