alled IMPs, for intermachine processors. The original IMPs were built by Bolt Beranek and Newman, a Cambridge, Mass., firm that also operated the ARPAnet's Network Control Center. The network's core protocol was called NCP, for Network Control Protocol. The TCP/IP protocol suite was developed in the 1970s, and NCP support was turned off in the IMPs in 1983. TCP/IP was important because it created two separate layers with distinct functions out of what had been NCP's function. IP ran in all of the IMPs, relaying packets across the network on a connectionless basis. Each packet stood independent of all others, with no prior connection between the two ends required, and the full source and destination addresses in each packet. (By analogy, postcards and let- ters travel the same way, albeit more slowly.) TCP only ran at each end of the network, in the host computers. It resequenced received packets (IP did not guarantee delivery order), detected dropped packets, and used retransmission to correct for lost or errored packets. This allowed the intermediate systems to be relatively simple; many functions only needed to be performed at the end points. The ARPAnet grew well into the 1980s but at one point was split into the Department of Defense's own internal MILNET and the more research- and academic-oriented ARPAnet. These interconnected networks became, in effect, the Internet, a network of networks. The ARPAnet itself was soon phased out, as the National Science Foundation took over funding of a new NSFnet back- bone, and other privately operated networks, such as CSNET, became part of the Internet. Other Packet-Switching Technologies Had Their Adherents Although TCP/IP has become the dominant form of data networking, it did not grow up in a vacuum; there were other packet-switching technologies that com- peted with it or were designed for different niche markets. One has to recall that in the 1970s and into the 1980s, most end users did not have computers on their desks; they had terminals. A good deal of computer 