I'm a complete noob when it comes to tethering, so i'm after a little info. Firstly why do carriers either charge extra OR do not allow tethering? I'm currently on O2 pay&go (i have been for many years) but to get unlimited internet, you have to pay for the high end tariff. I came across 'Giffgaff' (a network ran off the back of o2) and have just purchased a £5 'goodybag' which includes: 100 free minutes unlimited texts unlimited internet access BUT...it says in there terms and conditions that they do not allow tethering? Why do carriers do this, and how the hell do they know that you are tethering?