Loopback is the local-only network interface used for traffic that stays on the same machine. The best-known loopback address is 127.0.0.1 on IPv4, with ::1 on IPv6. When a service listens on loopback, it is reachable only from the host itself, not from other devices on the network.
In cyber security, loopback is a key boundary for private services such as admin panels, databases, and local AI inference servers. Defenders use loopback binding to reduce exposure, but that protection only works if the service is truly bound to localhost and not opened through a proxy, port forward, or misconfiguration. Attackers may try to reach loopback-only services through browser-based tricks like DNS rebinding, which can make a web page appear trusted while it probes private host services. Because of that, loopback should be paired with authentication and strict network controls.