Arpanet switched to TCP/IP twenty years ago, on January 1st, 1983. The protocol used by Arpanet before that date, the Network Control Protocol (NCP), allowed only 1000 computers to connect to the network. TCP/IP allowed more computers to connect to Arpanet's network address via routers.
Later in 1983, Arpanet split into ARPANET and MILNET. DNS came in 1984.
Those involved in the switch from NCP to TCP/IP sported t-shirts that proclaimed "I Survived the TCP/IP Transition". It's been suggested that the switch marked the official birthdate of the Internet.
Check out VNUNet's coverage here.