A network is a set of devices connected by communication links. The Internet today is made up of many wide and local area networks joined by connecting devices and switching stations. Most end users who want Internet connection use the services of Internet service providers (ISPs). There are backbone ISPs, regional ISPs, and local ISPs.
A protocol is a set of rules that governs communication. In protocol layering, we need to follow two principles to provide bidirectional communication. First, each layer needs to perform two opposite tasks. Second, two objects under each layer at both sides should be identical. TCP/IP is a hierarchical protocol suite made of five layers: application, transport, network, data-link, and physical.