Vint Cerf, at Stanford University, and Robert Kahn, at DARPA, published in 1974 'A Protocol for Packet Network Intercommunication', designing a protocol capable of connecting heterogeneous, independently managed computer networks — the central problem of what they call 'internetworking'. The ARPANET, existing since 1969, connected computers within a single packet-switched network, but there was no standard allowing different networks, with different technologies and administrators, to exchange information with each other reliably. The TCP/IP protocol solves this by splitting communication into two layers: IP (Internet Protocol) handles routing data packets between different networks, while TCP (Transmission Control Protocol) ensures those packets arrive complete, in order, and without errors, automatically retransmitting anything lost along the way. On January 1, 1983 — a date known as 'flag day' — ARPANET officially migrates to TCP/IP, making this protocol the universal standard on which the internet is built. Every subsequent internet service — email, the web, video streaming — runs on top of this foundational transport layer. Without TCP/IP, the infrastructure on which Tim Berners-Lee would build the World Wide Web fifteen years later would not exist.