ARPANET switches to TCP/IP, creating the internet
A single scheduled 'flag day' replaces the old network protocol for good
Quick facts
- Old protocol
- NCP (Network Control Program)
- New protocol
- TCP/IP
- Cutover date
- 1 January 1983
- Mandated by
- US Department of Defense, standard declared March 1982
What happened
ARPANET had run since 1969 on the Network Control Program (NCP), a protocol built only to connect ARPANET's own hosts, with no way to link it to other, separate networks. In March 1982, the US Department of Defense declared a new protocol suite, TCP/IP, its official standard and set a deadline: every host still using NCP after 1 January 1983 would lose access to the network entirely. Engineers coordinated a single global cutover, sometimes called 'flag day,' on that date, extensively tested in advance so the transition caused no major outages. TCP/IP's use of 32-bit addressing also allowed for roughly 4 billion possible host addresses, letting separate networks interconnect into one network of networks.
Why it matters
NCP could only ever be ARPANET's private internal protocol; TCP/IP was designed from the start to let independent networks connect to each other, which is the literal meaning of 'internetworking.' The 1 January 1983 cutover is the date many historians point to as the moment ARPANET truly became part of what we now call the internet.
How we know
The Internet Society's own report on the 1983 migration, based on a presentation by engineer Ron Broersma who participated in the transition, documents the DoD's 1982 standard declaration, the 1 January 1983 deadline, and the address-space consequences of the switch.
Sources
- Internet Society. Final report on TCP/IP migration in 1983 · General sourceinternetsociety.org · Cited as a "reference" source (no stronger domain match).
- History of Computer Communications (Gary C. Kessler). TCP/IP and XNS, 1981-1983 · General sourcehistoryofcomputercommunications.info · Cited as a "reference" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)
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