Some great answers above. Just to tidy up one detail though, hubs can have IP addresses if they are manageable; witness the old 3Com Dual Speed Hub 500 for example. But before you get confused, here's the key: switches and hubs don't need IP addresses to forward traffic and perform their network function, but they do need them if there is anything provided in the switch/hub that you can connect to remotely to perform some useful purpose such as switching ports on and off or monitoring the traffic levels
