There is a special network device, called the loopback device, to enable applications in the local computer to communicate. This device works at minimal cost and behaves like a physical network device. This is the reason it is often used to test network applications locally or to design them so that there will be no difference between situations where the communication partner is reachable locally or remotely over the network. One example is the X-Windows system. The loopback device in Linux is denoted lo and has the reserved IP address 127.0.0.1.