Generally speaking if you have your Wireshark configured to promiscuous mode it would gather all of the packets it can see. Switches are interesting because they try to only send packets to the port which the packets correspond to. If you can configure a spam port on your switch, the system connected to that port would be able to see every packet going through the switch. If you don't know how to do that or your switch doesn't support that, the alternative is to hook the systems to a hub. Hubs broadcast all of the packets to all ports, so any system would technically see all traffic. Hope this helps.