Each mobile device would have to have it's own unique identifier.
I don't know if IPs are handed out directly to mobile devices or if some other translated method it used.
They may even be assigned private IP addresses and not public IPs.
In any event, each device would need a unique address of some kind so that the device gets back the proper packets of information. Otherwise, if you and I were browsing from different devices, you might receive a web page that I requested. Imagine how embarrassing it would be for me when I go looking for birth control pills and they show up on your screen.

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