The Internet is a rapidly-growing worldwide network of computer networks that connects millions of users in almost all countries. It uses a standard addressing and communications system so that computers of different types can communicate with each other without having to store information about the structure of the network or about all the other computers on the network. This allows computers running different operating systems to all be part of the network. Most of the machines on the Internet use the Unix operating system, but you will see DOS and Macintosh machines, machines running VMS, and others, too. Before the creation of the Internet, computers located at different sites could only communicate if they were connected by phone line.
Many people still use ordinary phone lines to connect to the Internet.
However, large institutions usually run cables that connect directly to the Internet. It originally had four nodes and was designed as a decentralized military network that could withstand a nuclear war. Originally electronic mail services were only provided for testing purposes. But e-mail became unexpectedly popular very quickly. At that point the Internet had about 300,000 nodes.