Originally, networking was confined to very small setups, usually in one building. In fact, most networks started out as ways for terminals and other peripheral equipment to transmit data to the CPU, rather than for several different CPUs to share files or mail. These days were in the mid-1960s when large timesharing mainframes were being established and there was a need to connect hundreds of terminals to a central CPU. Then someone got the idea of sending data over telephone lines, so modems were invented to convert the computer's digital signals into analog signals that the telephone lines could handle.

Eventually, someone got the idea that CPUs might want to communicate directly with each other, especially long distance. The airline reservation system SABRE is an early example of a large distributed application where the "program" was never-ending and actually spread over many computers and terminal controllers over a wide geographic area.