Network frames always have a type field associated with them. These type fields vary in length, position and meaning by frame type (frame types generally correspond one-to-one with hardware types, but see "Ethernet Packet Types" below). The meanings of the type numbers are always carefully defined and every type number is registered with some official body. Do not use a type number which is not registered for any standard hardware you use or in a manner inconsistent with that registration. The type field allows the SANA-II device driver to fulfill CMD_READs based on the type of packet the driver user wants. Multiple protocols can therefore run over the same wire using the same driver without stepping on each other's toes. Packet types are specified as a long word. Unfortunately, the type field means different things on different wires. Driver users must allow their software to be configured with a SANA-II device name, unit number and the type number(s) used by the protocol stack with each device. This way, if new hardware becomes available, a hardware manufacturer can supply a listing of type assignments to configure pre-existing software.