The Utility Network is a local Ethernet, running between the various focal stations of the WHT, and the control room, over which the microprocessor systems (the 4ms systems which control the instruments and the Engineering Mimic; the VME computers which control the Autoguider, and the DMS; and the CCD controllers) communicate. Each device communicates via a Network Interface Unit (NIU), and each NIU can connect up to 4 devices to the Ethernet link. The system computer is also connected to the Utility Network via an NIU; this allows the transmission of messages from the User Interface to the devices. The Utility Network Ethernet is physically separate from the Ethernet which connects the VAXcluster and various other computers around the site. Utility Network messages conform to a local protocol set out in Johnson 1986 (Electronics report 412, 4.2m WHT Utility Network: Instrument Control Recommendations).