Note 26

At the time ARPA wrote the Request For Proposal for the network they envisioned an IMP adjacent to every Host, so the RFQ called for one Host interface per IMP. Almost as soon as the implementation began, ARPA requested BBN to make provision for multiple Hosts per IMP.  This was accommodated by adding a 2-bit Host address field and building some internal IMP tables to maintain state information for up to 4 Hosts, but the basic packet processing algorithms were not redesigned; thus the Hosts were tightly coupled in the packet processing algorithms. The original IMP-Host interface cable drivers were only rated for a 30-foot cable, but it was soon realized that an organization with multiple Hosts would be unlikely to have them all located within a 30-foot radius of an IMP location, so a “distant Host” interface was designed which allowed cable runs up to 2000 feet. Even this did not accommodate the needs of every organization so the Very Distant Host interface was devised to use a communication circuit rather than a multi-wire cable.  However through all these incremental changes the packet processing algorithms remained as they were first designed.  Finally the interference among Hosts of different capabilities became intolerable and in 1975 Q1 the software was reimplemented to decouple the processing of packets from multiple Hosts.