In Example 10–1, the IPv4 address 127.0.0.1 is the loopback address. The loopback address is the reserved network interface that is used by the local system to allow interprocess communication. This address enables the host to send packets to itself. The ifconfig command uses the loopback address for configuration and testing, as explained in Monitoring the Interface Configuration With the ifconfig Command. Every system on a TCP/IP network must use the IP address 127.0.0.1 for IPv4 loopback on the local host.