My understanding (for Ethernet) is this: An Ethernet Hub connects some computers using the Ethernet protocol. Any packet passed through the hub is replicated and sent out to all connected computers. The hub is kinda dumb, and doesn't use any information from the packet.
A Switch examines the Ethernet destination address and sends the packet out the appropriate port if it has seen a packet with that address before, otherwise it broadcasts. It also examines the source Ethernet MAC address to build up knowledge of what's connected to it. A Router examines the IP source and destination address..uses higher level information to be more precise. A Bridge connects two different types of networks...for example Ethernet and Token Ring. Tim Wright Assistant Lecturer Department of Computer Science University of Canterbury http://www.cosc.canterbury.ac.nz/~tnw13
