ARPANET switches to TCP/IP

Through the late 1970s ARPANET ran on NCP, an older protocol built for a single homogeneous network and unable to route traffic between separate networks. On 1 January 1983 every host on ARPANET switched over to TCP/IP at once, a coordinated cutover engineers nicknamed flag day because there was no way to run both protocols side by side during a gradual transition; hosts that were not ready lost connectivity until they upgraded. The switch mattered because it replaced a protocol tied to one specific network with one designed from the start to connect many independent networks, turning ARPANET into the technical backbone of what could now genuinely be called an internetwork.