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Networking Fundamentals · Foundational

Loopback Plug

A connector that routes a port's transmit pins back to its receive pins to test the port itself without a network.

Definition

Loopback plugs exist for RJ45, fiber and serial ports. They verify that a NIC or switch port can send and receive. They are physical Layer 1 tools, unlike the 127.0.0.1 software loopback.

Example

A technician inserts an RJ45 loopback plug and the NIC link light comes on.

Exam tip

A loopback plug tests the port hardware; pinging 127.0.0.1 tests the software stack.

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