[Rob]

A switch is a dumb device. It basically just allows you to connect cables, but does not know where to direct the traffic. AS far at the modem is concerned if you have both PCs plugged into the switch and the switch wired to the modem, the modem will be confused and unable to see both PCs.

A router will receive traffic to and from the modem. The modem will only see the router and thinks that the router is the single PC whcih it is expecting to talk to. A router will intercept traffic from the modem, work out which PC it is for and send it in the right direction.