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You should notice the message 'Disconnecting Nearby Wi-Fi Until Tomorrow' when you use the Control Center to turn it off. Turn off Wi-Fi and open Settings > Wi-Fi to check. Your iPhone will disconnect from the Wi-Fi router or hotspot and instead use mobile data temporarily. It will not turn off the Wi-Fi service itself, and after 24 hours, the iPhone or iPad will connect to that Wi-Fi network automatically. Turning off the Wi-Fi from the Control Center will only disconnect your smartphone from the connected Wi-Fi network/router for roughly a day. The Control Center was designed as an easy-access menu where you can toggle a bunch of services like Wi-Fi, Bluetooth, and torch on and off from anywhere on the screen. Realizing that toggling off the Wi-Fi didn't actually turn it off can be baffling how your battery drained gradually and how come the Wi-Fi network got turned on automatically. You may have turned off Wi-Fi to use cellular data or save battery juice while you're away from a known network. That's one reason why users are left wondering how their iOS Wi-Fi is turning on automatically. While Control Center makes it quick and easy to manage Wi-Fi and other connectivity options on your iPhone, most people don't fully grasp how it works.
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On your Apple iPhone or iPad, you can swipe up from any screen and toggle various services on/off. Apple added Control Center in iOS as part of a new quick menu in iOS 7.
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