How do I create an ad-hoc network on the Mac?
You can create a Wi-Fi network between two or more devices without using a Wi-Fi router. These networks are often referred to as ad-hoc networks. All you need is a computer running Mac OS X.
Choose Create Network from the AirPort status icon in the menu bar. If the icon isn't in the menu bar, choose Apple > System Preferences > Network > AirPort and select the "Show AirPort status in menu bar" checkbox.
Give the network a name and select a channel from the pop-up menu. You may use the default channel 11 but you can also choose any other channel.
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Now your iOS devices can join the network you've created by selecting it in the Wi-Fi section of the device settings
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After connecting your iOS device with the ad-hoc network give your computer some time to assign a valid internet address to your iOS device. This may take a few seconds.