What protocol did ARPANET use before TCP/IP?

Before TCP/IP, ARPANET used the Network Control Program, or NCP.

NCP was the protocol suite that let ARPANET host computers communicate before the network adopted TCP/IP. Its development was led at UCLA by Steve Crocker and other researchers, including Jon Postel. NCP was implemented around 1970 as ARPANET expanded beyond its earliest experimental connections.

NCP supported important early network services, including remote login and file transfer. However, it was designed primarily for communication within ARPANET. As researchers sought to connect separate networks—such as packet-radio, satellite, and other networks—a more flexible internetworking architecture was needed.

TCP/IP supplied that broader design. Version 4 of TCP/IP became the ARPANET’s production standard on January 1, 1983, in an event often called the “flag day.” NCP was then replaced for ordinary ARPANET communication. The change helped establish the Internet’s model of linking independent networks through common protocols.