A loopback address is an IP address that points back to the same host. The loopback address is used to test network interfaces and applications on a host without relying on external network connectivity. The most common loopback address is 127.0.0.1 for IPv4 and ::1 for IPv6. When a packet is sent to a loopback address, it is not forwarded by any network interface, but is processed locally by the host.