TCP describes a network of networks

By the early 1970s several separate packet networks existed, including ARPANET, radio-based networks and satellite links, but each used incompatible internal formats with no common way to talk to one another. Vint Cerf and Bob Kahn published a design describing gateway computers that could sit between different networks and translate their packets using a single shared protocol, without requiring every network to change its own internal technology. That gateway concept, refined over the following years into TCP and the separate IP, established the core principle that later let ARPANET, university networks and eventually millions of unrelated networks interconnect as one Internet rather than remaining isolated islands.