A loop back is exactly what it sounds like. It sends a signal in a loop. This is commonly used to test interfaces or Point-to-Point troubleshooting. For example, the loop back address on your PC is always 127.0.0.1. To do your own loop back test you could ping 127.0.0.1. If this fails then your network card is not installed correctly or bad.
Dig Deeper on Network Administration
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