My computer is already connected to a 100Mbps LAN. I can use wvdial to connect to internet using a modem when I have my LAN disconnected.
Now, I want to share this modem internet to one of the IP addresses available on LAN. Say 10.100.99.56 for example.
First of all, can it be done?
How do I go about doing that?
You need a router. You can configure your computer to act as a router (this is usually referred to as "connection sharing"); most OSes have some built-in mechanism to do this.
Or you could just buy a hardware router, and put it between your modem and your computer. This is probably both easier to set up and more flexible (as it will work even if your computer is off). Simple hardware routers (even with WLAN built in) are quite cheap, usually starting at about 30 $ / Euro.
Now, I want to share this modem internet to one of the ip address available on LAN say 10.100.99.56..
Normally, you'd make the internet connection available to all computers on the LAN. If you want to restrict it, that can usually configured on the router as well.
You can't do it without additional hardware. You can do it without a router, but you'll need a second network card in your PC so it can connect to the LAN and the modem at the same time. Then you'll need to set up internet connection sharing (not sure how hard this is to do on Linux) or set up a proxy server on your PC.
Ultimately though, it's really worth just spending a few bucks and getting a router. You'll save yourself a lot of time and hassle and ultimately have a better solution.
ipmasq is what you want. This tutorial is ubuntu-specific, and shows how to set up a dhcp-server if you don't want a static IP on your second computer.
