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The ARPANET was originally conceived to support high-data-rate distant communication between large mainframe computers. Because of the hardware limitations of the Honeywell 316/516 processor, which was selected as the original ARPANET interface message processor (IMP), most ARPANET IMPs are resticted to supporting a maximum of four host attachments (computers or gateways to other networks). As the ARPANET community expanded many sites experienced the need for more than four host ports. The response was to install additional IMPS. With the H316/516 no longer available and the C/30 IMP, being developed by the BBN Computer Company not expected to arrive until 1981. no more ARPANET community, hosts attached to IMPs have increasingly tended toward less powerful types, such as the DEC PDP-11 minicomputer. Many of these microcomputer hosts require only moderate data-rate network communication or occasional access. Newer node-switching equipment utilizes the Bolt Beranek and Newman Pluribus, which eliminates hardware constraints upon the number of host attachments. Nevertheless, the large majority of deployed IMPs are still of the older Honeywell type. The desire to attach a large number of minicomputers or microcomputers to the ARPANET induced DARPA to assign SRI the task of developing the port expander concept into a working product. This manual describes the functions, installations, and operation of the port expander.