NSFNET was just a long-haul backbone, part of the underlying physical network at the time. The Internet is really defined by the inter-networking protocols, si

NSFNET was just a long-haul backbone, part of the underlying physical network at the time. The Internet is really defined by the inter-networking protocols, since the very term "internet" implies it can ride on different physical networks. To be a part of the Internet you just have to speak the TCP/IP protocol stack. The same wikipedia page points out these were standardized around 1982.