Loopback (also written loop-back) is the routing of electronic signals or digital data streams back to their source without intentional processing or modification.
A patch cable may also function as loopback, when applied manually or automatically, remotely or locally, facilitating a loop-back test.
In telecommunications, loopback, or a loop, is a hardware or software method which feeds a received signal or data back to the sender.
As a test, many data communication devices can be configured to send specific patterns (such as all ones) on an interface and can detect the reception of this signal on the same port.
Media such as optical fiber or coaxial cable, which have separate transmit and receive connectors, can simply be looped together with a single strand of the appropriate medium.
It utilizes a terminal emulator application to send characters, with flow control set to off, to the serial port and receive the same back.
Any traffic that a computer program sends to a loopback IP address is simply and immediately passed back up the network software stack as if it had been received from another device.
An exceptional (non-loopback) use of 127.0.0.0/8 network addresses is in Multiprotocol Label Switching (MPLS) traceroute error detection, in which their property of not being routable provides a convenient means to avoid delivery of faulty packets to end users.
Unlike physical loopbacks, this does not involve double analog/digital conversion and no disruption is caused by hardware malfunctions.