Telnet turned remote terminal access into an interoperable Internet service by defining a Network Virtual Terminal and a negotiated command language that could bridge unlike hosts and terminals.
FTP evolved from the early ARPANET into a long-lived Internet protocol for transferring remote files, standardizing authentication, directory commands and separate control and data connections.
ARPANET linked research computers through packet-switching nodes, created a community for network protocols, and provided the experimental environment from which Internet architecture emerged.
Ray Tomlinson's ARPANET mail experiment joined local mailbox software with network file transfer and established the user@host addressing convention that became a universal symbol of online identity.