How do I share my Ubuntu's Internet connection with a LAN (via hardware router)

I know this sort of question has been asked many times, and I've thus tried many ways to get it working, but I think my setup is a bit different from what I've seen others trying. I've got an Ubuntu box with a usb mobile broadband modem that connects me to the Internet, and I'm trying to share this Internet connection with an ordinary linksys router (WRT120N), and thereby with my wifi LAN. I've connected my Ubuntu box to the router via ethernet, but don't know how to get the router to use my Internet connection. Maybe this is more of a problem with the router configuration, but I tried creating a new wired connection in Ubuntu and selecting "Shared to Other Computers" under method, which seems useless.

Is there some easy way to make the router use the Internet connection used by the Ubuntu box for the LAN?

Here's how I did it:

Equipment:

• Ubuntu 11.04 Desktop on 1st Gen. eeePC

• Linksys WRT120N Wireless N Router

• Huawei E620 USB Broadband Modem

• Ethernet cable

Hardware Setup:

• Huawei plugged into eeePC

• Ethernet cable plugged into Ethernet port

• other end of Ethernet cable plugged into the Internet port of The Linksys

Ubuntu Setup:

1. connect the Huawei to the Internet
2. disconnect the wired connection (Auto Eth0) in network tray icon
3. create new wired connection (Edit Connections... > Add, enter a name) and select Shared To Other Computers from IPv4 settings tab.
4. connect to this new wired connection
5. note down the settings for this connection under Connection Information, and the DNS servers for the Huawei connection

Linksys Setup:

1. connect a computer on the receiving side to the Linksys (wired or wireless)
2. open a browser in this computer, navigate to Linksys (192.168.1.1 is default, user and password should be both admin)
3. Change Internet Connection Type to Static IP
4. Under Internet IP Address I put the number that came after the eeePC's IP address for this connection (again, not the IP for the Huawei connection), this was just a guess as I'm pretty clueless about how this works. So, my Ubuntu machine had an IP of 10.42.43.1, and I gave the Linksys 10.42.43.2
5. Subnet Mask should be same as the eeePC, 255.255.255.0
6. Default Gateway should be the eeePC's IP address, 10.42.43.1 in my case.
7. DNS 1 (etc.) should be the same as the Primary DNS (etc.) used by the eeePC in the Huawei connection.
8. Click Save Settings.

And now all machines on my LAN can connect to the eeePC's Internet via the linksys router. All is well.

It's very easy. I use Norwegian, so the translations might not be correct, but I hope and believe you'll understand.

Click on the network icon and select "edit connections". Select wired connections in the first tab and select your network, usually something like "Auto eth0". Click the edit button on the right and in the new window, look at IP settings. There is a combobox there where you can choose "method" that includes DHCP, Static and Shared with other computers, among others. Choose "shared with other computers".

Now, simply connect your wired network connection on your PC to the uplink-port on your router (usually called WAN).

Finished. :)