What is the purpose of a loopback test?

• A. To test external connectivity to another host
• B. To verify that a network interface can send and receive on itself
• C. To measure throughput between two devices
• D. To verify routing table correctness

Answer: (D)

The loopback test checks the ability of the local machine to send data to itself and receive it back through its own networking stack. It uses the loopback address (like 127.0.0.1) so packets never leave the host, allowing you to verify that the operating system’s network software, drivers, and internal routing of the NIC are functioning correctly. This helps confirm the local TCP/IP stack is working without involving any physical network hardware.

It does not test connectivity to another host, so external reachability is not assessed. It isn’t about measuring throughput between devices, since there’s no other device involved. It also doesn’t verify routing table correctness, which is about how the system forwards traffic to other networks rather than handling local loopback traffic.