On January 1, 1983, in what is known as "Flag Day", ARPANET completes the mandatory transition to the TCP/IP protocol suite, designed by Vint Cerf and Robert Kahn in 1974, replacing NCP, which did not allow interconnection with external networks. All network nodes had to update their software simultaneously with no gradual transition period. This event marks the operational birth of the Internet as a network of networks: TCP/IP separates the transport layer from the physical layer, allowing heterogeneous networks — including Ethernet — to communicate via a common protocol, laying the open, interoperable architecture on which commercial Internet expansion would be built the following decade.