If its a usb modem, then you cant... You should try to get a ethernet version of it, but still dont worry.. All new motherboards//computers have atleast 2 usb ports ;-)
But still my personally opinion is that ethernet is more reliable and more compitable... usb modems are very hard to get working with alternative oses like amigaOS, linux and so on..... Since there is lack of drivers... While ethernet dosent need any other driver than the one for your ethernet card.