Open book tests and e-books
I'm of the opinion that internet access should be absolutely forbidden or impossible during exams. It's a can of worms that is best left tightly sealed.

Every modern tablet computer, and most e-readers, have Wi-Fi capability – therefore internet access. Really, it only makes sense to allow these if it is non-trivial to access the internet and if you have sufficient deterrents in place for the more determined cheaters.


If the exams are held in a lecture block that is isolated from any offices or research labs, it should be feasible to request that the Wi-Fi network is turned off for the duration of the exam.
Some tablets and e-readers have cellular network capabilities. For an exam with many students, this might be difficult to police. But if you check the model of each tablet or e-reader on entry to the exam and remove the SIM cards, that will remove their internet access. Unless the student puts a fake, inactive one in the device and keeps their active card in their pocket, that is.
The Kindle 3G e-ink readers (perhaps others, too) do not have user-accessible SIM cards. Sure, web browsing on them is a pain and limited to 50MB / month – but you can still have email or webchat conversations with a friend on their computer at home who is telling you the answers. You cannot reasonably ban Kindle 3G readers but allow iPads.
Anything with Wi-Fi capability can connect to an ad-hoc Wi-Fi network that someone created with their cellphone hidden in their bag that they left at the front of the room. This is absurdly easy to set up, though it is also possible to detect hidden SSIDs which would alert you to a possible cheater.


As the years pass, although it does seem increasingly unreasonable to disallow e-readers into an open book exam, allowing networked devices owned and controlled by the students will make cheating easier. All we can do is make it hard enough to deter casual offending. A determined cheater will always find a way.

Other than locking everyone inside a Faraday cage, you simply cannot stop people who have their own devices from accessing the internet. You do your best to find them, though!

Other mitigation strategies might be:


Increased presence of invigilators. If the exam is held either in a room with individual desks, or in a lecture hall with every second row empty, it is easier for the invigilators to see up close what everyone is doing. If there were 6 to 10 invigilators in an exam hall of 100 students, as opposed to the usual 2, that might be a sufficient deterrent.
Use electronic surveillance / prevention, such as a PocketHound.
Ban all electronic devices, but have enough copies of the course textbook in the library so that anyone who does not have one can borrow a copy. This might get expensive for a large class!
Hand out university-provided e-readers to everyone without a paper copy. If the network capabilities are locked down under a password, this should be safe. Again, not a cheap or easy option.
Only allow one or several pages of handwritten notes into the exam. In most subjects, spending too much time flipping through a textbook during the exam is a waste of time and an indication of a lack of study. The time spent in carefully curating a "cheat sheet" is usually worth far more in the exam than the cheat sheet itself. Is a full open book policy really necessary? If every question needs the student to refer to a textbook, perhaps the exam isn't asking the right sort of questions.