Which IP address is the IPv4 loopback address?

• A. 0.0.0.0
• B. 127.0.0.1
• C. 255.0.0.0
• D. 192.168.0.1

Answer: (B)

The essential idea is that the loopback address is a special IPv4 address used to reach the local machine itself, without sending packets onto the network. It lives in the 127.0.0.0/8 block, and the most commonly used one is 127.0.0.1, often called localhost. When you send traffic to this address, the operating system loops it back to its own network stack, so you can test software and services on the same host without any real network transmission.

So, 127.0.0.1 is the loopback address because it’s part of that 127.* range and is the standard address used for this self-referential testing. The other addresses serve different purposes: 0.0.0.0 is the unspecified address used in contexts like boot or binding to all interfaces, not a destination for loopback; 255.0.0.0 is a subnet mask used in routing, not a host address; and 192.168.0.1 is a private LAN address, not a loopback address.