The 1st of January, 1983 is often referred to as 'the day the Internet was born', you may ask why is that? It was the day that ARPANET officially switched from using the NCP (Network Control Program) to using TCP/IP. TCP/IP is the network protocol (software) that the Internet runs on. ARPANET was the first packet switching network developed in the United States, and it was launched in 1969. The Network Control Protocol (NCP) was the network protocol deployed on ARPANET, and was developed/deployed by the Network Working Group (NWG) and outlined in RFC33 by S. Crocker, S. Carr, and V. Cerf. RFC33 notes other scientists who helped design NCP: J. Rulifson, W. Duvall, G. Deloche, J. Curry, P. Rovner and W. Crowther. The Network Control Protocol (NCP) was the software that provided a format for network data and how the transmission of this data was sent from host to host across ARPANET.
Efforts to improve upon the Network Control Protocol (NCP) began nearly as early as the software was deployed, led by Vint Cerf and Bob Kahn. They envisaged an open and simple protocol that would make it easier for networks to connect to one another. The process of refining TCP/IP - two separate protocol named TCP and IP - lasted nearly a decade: from the early 1970's to the early 1980's. By the early 1980's, with research prototypes tested, it was concluded that TCP/IP was ready to become a network standard. Jon Postel, one of the founding fathers of the Internet, sent an email to the network nodes of ARPANET on the 14th of October, 1981: the subject of the email was as follows "It is really very important for everyone to notice the deadline for completing the cutover to IP/TCP and the elimination of NCP from use in the ARPANET. The deadline is: 1 January 1983. That is 14 and a half months from now. Really not much more than a year."
The switchover was not seamless - even though Jon Postel gave the network nodes plenty of notice - as some nodes did not update their networks and were offline for months after the switchover. TCP/IP has been developed to the present day by the following organisations: Internet Society, IAB, IESG, IETF, IRSG and IRTF. TCP/IP enabled more networks to be interconnected throughout the 1980's, and this system of interconnected networks were referred to as the Internet by the early 1990's. The 1st of January, 1983, was the clearest signpost when the older packet switching networks evolved to become the Internet.