[ubuntu] Spring u760 modem. how share internet connection to mac book over LAN

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http://ubuntuforums.org – I have a PC with Ubuntu 9.10. It gets its Internet from a USB Sprint U760 broadband modem. the PC is connected by ethernet to a netgear router which is connected by ethernet to an MAC Airport Router. I have a macbook laptop that connects to the airport through wifi. I want to set the PC up as a gateway to share the broadband cards connection with the macbook. I know that I can resolve this by purchasing a wifi router compatible with a usb modem, but is there any way to make this work with out purchasing yet another router? I have attempted to use Firestarter to do this but it doesn't seem to recognize the broadband USB modem as a connection and thus doesn't share it. please help. I am a bit new to Ubuntu/linux but do have some computer skills. thanks for help in advance. Murefu (HowTos)