Princeton University's Office of Information Technology is now suggesting that the iPad's DHCP lease renewal problem may be related to the iPad's screen being locked, either manually or automatically. The best workaround seems to be to disable automatic screen locking and avoid locking the screen manually; the downside to this is of course reduced battery life. Other (but worse) possibilities include turning off Wi-Fi or the entire iPad at the end of each session.follow link
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Princeton Suggests Workaround for iPad DHCP Flaw