Trying to setup a gateway via pppoe

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http://askubuntu.com – I just installed Ubuntu 12.04 on a computer. I want to use this computer as a router for my other computers. The computer only has 1 network card, there is no slot for a second card. I can get the computer to connect to my ISP via pppoe and it can access the web. However the other computers on my local net can only see the router computer. I have tried 2 different pppoe packages, roaring penguin and network-manager-gnome. When I get it to work I will be replacing a 15 year old computer that is running Slackware 7.1. The old computer is currently my router. It is running Roaring Penguin> I tr (HowTos)