Loopback Plug for Verifying NIC Send and Receive
Short answer
A A loopback plug ties the NIC port's transmit pins to its receive pins so data sent out is echoed back, verifying the port can both send and receive.
Question
Key Takeaways
The question tests knowledge of network test tools: the trap is choosing a cable tester, which only verifies cable continuity and wire mapping, not the NIC's ability to transmit and receive data end-to-end.
A loopback plug is a small connector that physically ties a NIC's transmit pins to its receive pins, so data sent out of the port is immediately echoed back. This page establishes that the loopback plug is the tool used to verify whether a NIC port can actually send and receive frames, distinguishing it from cable testers, toner probes, and network taps.
Common mistake: The most common wrong answer is C, cable tester, because learners associate it with "testing the port." A cable tester checks the cable's pin-to-pin wiring and continuity only; it says nothing about whether the NIC's transmitter and receiver circuitry can send and receive frames.
Expert Analysis
Why the Answer Is Correct
A loopback plug (option A) is a pass-through connector that shorts a port's transmit pair to its receive pair, causing every frame the NIC sends to be immediately returned to its own receive buffer. When you run a loopback test on the NIC's loopback adapter or attach a hardware loopback plug to an RJ-45 port, successful send-and-receive confirms the adapter, driver, and port are functioning. This is the standard way to isolate a NIC fault from a cable or switch fault.
Why the Other Options Are Wrong
A toner probe (B), also called a wire tracker, is used only to locate and trace a specific cable within a bundle by emitting and detecting an audible tone; it tests no data transfer. A cable tester (C) checks continuity and correct pin mapping across a patch cable or installed run but cannot exercise the NIC's own transmit/receive electronics. A network tap (D) is a passive or active device that mirrors live traffic for monitoring and analysis; it observes traffic already flowing rather than proving the port can generate and accept its own frames.
Community Comment Notes
The lone comment votes for A, and the reasoning it gives matches the technical standard: "Loopback sends data OUT of the NIC and BACK IN to test sending and receiving." No commenter defended the cable tester, and the vote reasoning is consistent with CompTIA's networking-tools objectives, so the community position aligns with the analysis here.
Official Reference
https://www.comptia.org/training/resources/exam-objectives
This link opens the official vendor documentation in a new tab.
Exam Strategy
Read which layer the tool operates on: anything that checks wiring or locates a cable (toner, cable tester) cannot prove the NIC works, while the loopback plug exercises the NIC itself. In troubleshooting flows, a passed loopback test lets you rule out the NIC and move on to the cable, patch panel, or switch port.
Frequently Asked Questions
Why is a cable tester not enough to verify a NIC port's send and receive?
When would I use a toner probe versus a loopback plug?
Discussion 1 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 1
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acf9805 👍 1 Answered A499. A — Loopback plug 🧸 Loopback sends data OUT of the NIC and BACK IN to test sending and receiving. 🔁
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