Originally Posted by The_Chief
Is this saying that the laptop has a different IP address than the phone (I understand that); and even if the laptop is connected to the phone's internet, Verizon can see TWO discrete IP addresses piggy-backing in the same data packets, and therefore they know that multiple devices are using the same mobile internet connection?
Because this - bless marctronixx's heart - is confusing to me. Doesn't take much, you know!
Ok - there are several technologies at play here - I'll try to break them down.
1. Your phone and the internet
Methods vary by carrier. In the case of Sprint, all internet access goes through their proxy servers (centralized hub for routing internet traffic from phones to internet and back, Marc was referring to that example). Whether a proxy server is used or not, I'm sure we can all agree on the concept - the carrier knows what goes to your phone by definition.
2. Your phone and the carrier
Your phone will get an IP address from the carrier - they are acting as your ISP when you get the internet through them, after all.
3. Tethered devices
Your tethered devices don't get their own IP, recognized by the carrier, unless using carrier software (and even then, methods vary, so that's a maybe).
So basically, your phone starts doing the same thing that your home wifi router is doing - serving private addresses from one known public address.
And just as the cable or phone ISPs can determine how many devices are on a router if they want to, so can cell carriers.
The router tech is called Network Address Translation (NAT) - translating from one public address at the router point to individual addresses behind it. There's a data marker that the router tracks to know which internet message goes where - your PC or your kid's for example, with both running at the same time.
NAT packets can be detected.
The alternative to NAT is to set up the routing point - oh, I don't know, let's say a phone - as a proxy server - just like some carriers use. That's not easily detected, and was in the blogosphere this last year as such, for example, mentioned on XDA. But that was specialized software, I don't know if it's even still around. Whatever you do, don't search for FoxFi or its proxy add-on. Probably won't work.
Please do not violate your carrier terms of service - I'm sure no one here will do that. So, I don't think I need to monitor this much at all.