On January 1, 1983, the Defense Advanced Research Projects Agency (DARPA) officially required all host computers connected to the ARPANET to switch their core communication protocol from the original Network Control Program (NCP) to the Transmission Control Protocol/Internet Protocol (TCP/IP). Known as "Flag Day," this mandatory cutover had been carefully planned for years under the leadership of Vinton Cerf and Robert Kahn, who designed TCP/IP to allow heterogeneous networks to interconnect seamlessly.
Prior to Flag Day, NCP only managed communications between nodes on the single ARPANET network architecture. As packet radio networks (PRNET) and satellite networks (SATNET) emerged, a universal protocol suite was essential to unite these disparate networks into a unified "internetwork." Systems that failed to convert by midnight on December 31, 1982, were disconnected from the network until their software stacks were updated.
The transition was remarkably successful, establishing TCP/IP as the standard language of computer networking. This migration marks the technical birth of the global Internet, replacing a centralized research network with an open, scalable framework capable of spanning the globe.
Why it matters
Flag Day marked the official technical birth of the modern Internet. By replacing the limited Network Control Program (NCP) with TCP/IP, DARPA transitioned computer networking from a single experimental network into an open, expandable 'internetwork' of networks. TCP/IP's modular design enabled the seamless integration of diverse hardware, software, and transmission media, paving the way for global digital connectivity.
Interesting fact
To test host readiness prior to Flag Day, network manager Jon Postel executed planned 'pings' and temporary network shutdowns in 1982 to catch unready system administrators, and participants who successfully transitioned were given pin buttons reading 'I survived the TCP/IP switch.'
People
- Vinton CerfCo-designer of TCP/IP and DARPA Program Manager
- Robert E. KahnCo-designer of TCP/IP and DARPA Information Processing Techniques Office Director
- Jon PostelARPANET RFC Editor and switchover coordinator
- Dan LynchARPANET Manager who coordinated the Flag Day migration
Quotes
"On 1 January 1983, all hosts on the ARPANET were required to convert to TCP/IP. This was a true 'flag day' conversion."
Gallery
Location
DARPA Headquarters / BBN Technologies · Arlington · United States
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