The 5-4-3 rule is applied to
repeaters in
computer networking. It states that you can only have five
segments of
cable, four repeaters, and only three
hosts on the segments. Typically your first and last segment will contain a host, plus one somewhere in the middle; the other two will only contain repeaters to increase the distance you can trasmit a
signal. For example: say you have two computers that you want to connect, but they are 1000 meters apart and your cabling will only sustain a signal over 200 meters. You could use four repeaters to transmit the signal as diagramed below:
Host-----R-----R/Host-----R-----R-----Host
Each "-----" represents 200 meters of cable, and the Rs represent repeaters. Because the signal would be regenerated (if it's a digital repeater) or amplified (if it's an analog repeater) you could transmit the data the entire 1000 meters.
You can only have four repeaters because when the source computer sends the signal initially, it expects to get a response in a certain amount of time. If it doesn't, it will re-send the signal even though it allready arrived at the destination. Also, it wouldn't be able to tell if something else was being transmitted along the wire, so collisions would occur.