Packet switching soon found favor beyond the confines of ARPANET. Roberts left ARPA in 1972 to become president of one of the first companies to offer networking services to commercial customers. Several European countries had become interested in computer networking, and the U.S. government had other packet-based projects under way. Although ARPANET was presumably destined to remain a well-guarded dominion of computer scientists, some widening of its reach by connecting with other networks seemed both desirable and inevitable.

It was clear to Robert Kahn, who had headed the BBN design team, that network-to-network linkages would require an acceptance of diversity, since ARPANET's specifications for packet sizes, delivery rates, and other features of data flow were not a standard. Commonality would instead be imposed in the form of shared rules, or protocols, for communication—some of the rules to apply to the networks themselves, others meant for gateways that would be placed between networks. The job of these gateways, called routers, would be to control traffic, nothing more. What was inside the packets wouldn't matter.

To grapple with the various issues, Kahn joined forces with Vinton Cerf, who had been involved in designing the ARPANET protocols for host computers and also had experience with time-sharing systems on the ARPANET. By mid-1974 their recommendations for an overall network-to-network architecture had been accepted. Negotiations to finalize the two sets of rules, jointly known as TCP/IP (transmission control protocol/ internet protocol), took several more years, and ARPANET did not formally incorporate the new system until 1983. By then ARPA—now known as DARPA, the "D" having been added to signal a clearer focus on defense—was looking for release from its network responsibilities.

An exit presented itself in mid-decade when another U.S. government entity, the National Science Foundation (NSF), began building five supercomputing centers around the country, along with a connecting backbone of lines that were about 25 times faster than ARPANET's. At that time, research scientists of all kinds were clamoring for network access to allow the kind of easy communication and collaboration that ARPANET users had long enjoyed. NSF answered the need by helping to create a number of regional networks, then joining them together by means of the supercomputer backbone. Many foreign networks were connected. In the late 1980s ARPANET began attaching its sites to the system, and in 1990 the granddaddy of packet-switching networks was decommissioned.