Help - Need to setup up a small network without gui tools

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http://www.linuxquestions.org – I need to set up a small network but without using any gui tools. My network consists of 4 PCs connected through a switch. One of the PCs is also connected to the internet through a USB CDMA1X EV-DO modem (ppp interface). Now, I have designated this PC to be the default gateway for all the other machines. But I don't really know how to route traffic from the LAN to the internet via this PC and configure NAT. I have looked up at many tutorials but haven't managed to get it working. Please advice. (HowTos)