If you have a Cable modem that is capable of making an ethernet connection, all you will need is a router.  With a router, you can share the internet connection and also have a local network over which you can share files and printers.  If your 6400 has a wireless network adapter and you want to share the internet over wireless, then you would need a wireless router.  The other computer could still connect to the router by ethernet if it is located near the cable modem and router.  If your cable modem can only connect by USB, you would need a new modem, since it is next to impossible to find a router that can connect to a modem by USB.

 

Steve