The Internet is a rapidly-growing worldwide network of computer networks that connects over 20 million users in 50 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. 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 email became unexpectedly popular very quickly. At that point the Internet had about 300,000 nodes.