At the same time, the IMP also served as interface for the mainframe. Since only the IMPs were interconnected in a network this was also called IMP-subnet. For the first connections between the computers the Network Working Group developed the Network Control Protocal (Network Control Program]. Later on, the NCP was replaced by the more efficient Transmission Control Protocol. The specific feature of the TCP is the verification of the file transfer. Let’s take a short detour to England.