Before TCP/IP was adopted, the ARPANET used the Network Control Protocol, usually abbreviated NCP. It provided the host-to-host communication services that allowed computers connected to ARPANET to exchange data.
NCP emerged from work by the ARPANET Network Working Group, led by researchers including Steve Crocker and Jon Postel. It was implemented on host computers while the lower network layers were handled by Interface Message Processors, or IMPs. Applications such as Telnet and FTP could then operate over the resulting host-to-host connections.
NCP was not the same thing as modern IP. It handled reliable, flow-controlled communication within the ARPANET, while TCP/IP was designed to connect multiple distinct networks into an internetwork. That broader design was essential as networking expanded beyond the original ARPANET.
On January 1, 1983, known as the flag day, ARPANET officially switched from NCP to TCP/IP. This transition is often treated as a major milestone in the development of the modern Internet.