I believe the switch separates each port into a collision domain. It's a smart device which maintains a MAC address table so that the next time a message comes into the switch with a destination mac address that is in the table for one of the ports, it will only send it to that port rather than flooding all connected ports/devices (broadcast). This, of course, reduces the traffic on the network. A hub, on the other hand, will floods the data out of all ports. I like the link Matty - very nice! I hope these posts help you out.