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.
Now your iOS devices can join the network you've created by selecting it in the Wi-Fi section of the device settings
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.