ARPANET sends its first host-to-host message

Before this test, separate computer labs had no shared way to exchange data beyond dedicated point-to-point lines built for a single purpose. A UCLA student attempted to log into a Stanford Research Institute computer over the new ARPANET; the system crashed after only the letters L and O arrived, making "LO" the accidental first message ever sent across a packet-switched network. Engineers restored the connection within the hour and completed the login. The exchange mattered because it showed that Interface Message Processors, the specialized routing computers ARPANET used to move data in small packets, could carry a real interactive session between two physically separate machines rather than only laboratory test traffic. That proof pushed ARPANET from a funded research proposal toward a working four-node network that expanded through the 1970s.