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Imagine the following scenario, which I am proposing to a client for their service technicians' trucks.

I have a very small wireless LAN that exists only for the purpose of providing print services for iOS devices (using a Lantronix xPrintServer, a portable WiFi router, and a mobile printer, all wired into the truck's electrical system).

I have an iPad that is connected to the WiFi in the truck, and also has 3G service.

On that iPad, I go to check my email. What happens? Does it try to connect to the mail server over WiFi and fail? Or does it know it can't route that way and use the 3G instead?

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in my experience, if my phone connects to WiFi, all data goes over the WiFi even if it doesn't have an external route –  Jason Feb 10 '12 at 15:45

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When connected to both Wi-Fi and 3G, you have two IP addresses. One handed out by DHCP on your portable Wi-Fi network and one from AT&T (or Verizon, as may be the case).

To my knowledge and from my experience, your iPad will use the IP address obtained by Wi-Fi first, and then 3G. So if you're connected to a Wi-Fi network that doesn't have internet connectivity, the email address you're trying to check has nowhere to go since you don't have internet service connected to your portable Wi-Fi network.

Unfortunately, this means that in order to check your email, you'll have to disconnect from the portable Wi-Fi network, allowing the 3G IP address to become active.

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Yeah, that's what I thought. I'm going to propose that each truck get a wifi router that can pick up internet over 3g, so their wifi service is also their route to the internet. They can get cheaper iPads that way too. –  Dan Ray Feb 10 '12 at 21:59
    
Great idea! Good luck. –  Matt Love Feb 11 '12 at 11:44

I have a similar setup in this question, but I sort it by turning off the DHCP Gateway on my router. When a device attaches it knows it's not going to get internet access from the Wifi so it should fall back to using 3G.

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