According to the terminology of APPA network, forwarding nodes are collectively called InterfaceMessageProcessor (IMP). IMP is a computer dedicated to communication, which is generally a network device such as a switch and a router.

The elements of computer network can be divided into two categories: network nodes and communication links. Network nodes are divided into end nodes and forwarding nodes. End nodes refer to source nodes and destination nodes, such as user hosts and user terminals; Forwarding nodes refer to nodes that control and forward information in the process of network communication, such as switches and routers. Communication link is a channel for transmitting information between nodes. Transmission media mainly include telephone line, twisted pair, optical fiber and microwave. The computer network formed by connecting network nodes through communication links is shown in the following figure.