Let’s assess the situation: You have a 3G phone which allows tethering, a windows machine, a wireless access point and another PC that is connected to the LAN port of the wireless access point (or typically called a wireless router). The wireless access point is a home network and your ISP decides to disconnect your signal or is experiencing some problems. How do you share your 3G connection, so that other computers can connect through the wireless AP and use your cell phone’s 3G connection? It’s actually quite simple.

Tether your smartphone (in this case let’s say blackberry). Blackberry tethering in Windows over AT&T is allowed using a software called AT&T Communication Manager. Install ACM, and connect your Windows machine using a mini-USB cable to your phone. Goto Start->Run->cmd.exe. Type ipconfig /all to see the IP address and the DNS servers IP addresses.

Now connect the WAN link of your Wireless access point to this Windows machine’s ethernet port. Setup a static IP for this Windows machine say with a netmask of Now go to Network connections (from Control Panel), right click on the Mobile connection representing your blackberry, click on Advanced. In the Internet Connection Sharing section, check the box that says “Allow other network users to connect through this computer’s Internet connection” and select “Local Area Connection” (this is the same connection you connected to the WAN port of the wireless AP). Click OK. You may have to disconnect and reconnect your ACM connection to allow the settings to take effect.

Now that this is done, connect to the administration interface of the wireless access point to the other PC that is connected to the LAN port (or through the wireless) to the access point. Go to the administration interface of the wireless AP, and set a static IP for the router in the same subnet as with the Windows box (the one you set with Set the static IP on the AP to be say (remember this has to be the same subnet), netmask as and then the most important, the default gateway to (the IP of the windows box on the ethernet card). To set the DNS server addresses, use the same addresses you found using ipconfig /all in the first step. Otherwise you could also use open DNS servers or any other DNS servers but it’ll be best to use the DNS servers pointing to the ones used by the tethered connection because you can rule out DNS issues if something isn’t working and it comes down to troubleshooting. Once on the router, the static is set, the gateway is set, the DNS is set, you should be able to connect from your wireless network to the internet through your 3G connection! 🙂
Happy internet sharing! 🙂
Here is a schematic diagram:

[tethering]                [static IP]    [static IP]   [internal IP]  [DHCP address]
                                   |         |                |           |
[ phone ] <==> [Windows machine]:eth0 <==> wan:[Wireless AP]:lan <==> [client]