I'll always be disappointed with Windows Networking (poorly engineered - slow, cumbersome to work with at times). But I'll throw this question out anyway.
Have a LAN connected to Comcast for Internet. When Comcast goes out briefly, I'll use my iPhone as a modem. It is a little frustrating though that it seems initially, I have to disable my LAN card to force it to use the iPhone. Seems Windows takes a while or is unreliable as far as detecting the Internet is out on one connection and automatically route through the other.
What I'd really like is when Comcast goes out, and my iPhone is plugged in and sharing it's Internet connection, that my PC would share that connection with the other PCs on the LAN (3 others). They (2 are Win 7 including host, other is Vista, other is XP). I played around briefly and of course couldn't get it to work despite getting the Internet Connection Device to show up on the network - couldn't get the other PCs to router through it. I'm guessing they are all still routing through the router's WAN port.
I want something that does this AUTOMATICALLY where I don't have to manually make changes each time. Probably just a dream. At least in the Windows World.
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It would need alot of messing about. Your other Pcs currently look to your ComCast for thier IP address and default gateway etc. You would need to disable the comcasts DCHP ability, then configure all the other machines to point to the IP address of the machine sharing its internet instead. Massive pain. However there might be another option. You could make all the machines wireless, and as long as they are within range of the machine you want to use the Iphone with, you could download connectify-me (only works with win7) and set your PC up as a wireless hotspot and share your Iphones internet that way. Or even better I believe there is a app which allows you to tether to your Iphone wirelessly anyway.