The connected collection of millions of networks is known as the Internet . A pair of hosts is joined by a communication link which may be a cable, a wire or a radio frequency. Messages between hosts have to be passed along a path of communication links, from machine to machine. The Internet is conceptually divided into Protocol Layers . The highest is the application layer, where humans meet software.
Hosts can communicate because they agree to share common protocols .
Different protocols are needed at different layers. These protocols are nearly always defined in terms of the client , which initiates communication and requests some service, and the server which waits to be contacted, and then provides the service. The client is on one host, and the server is on another host. The Internet is fast and inexpensive because the major protocols adopt a packet switching strategy. The early Internet was the first packet-switched network.