Each iOS device has a Unique Device Identifier — a predesignated sequence of 40 characters, each of which is arranged in a unique way. Think of UDID as your phone’s social security number: They’re used by developers to connect specific devices to iOS Developer Accounts, allowing them access to test exclusive content without the threat of outside interference.
Developers typically perform a closed beta before a major release to test the functionality of iOS apps on different devices. UDID’s allow them to hand-select particular devices in a way that’s fast, painless and secure. To start an open or closed beta for an app of your own, you’ll first need to create an Apple Developer’s Account: Click here for a quick how-to!
To find your UDID, open up the latest version of iTunes (http://www.apple.com/itunes/) and connect your iOS device to your computer
Select your iOS device by clicking the device’s image located at the upper-left corner of iTunes’s UI
On the next screen, a window should appear listing your phone’s Capacity, Phone Number, and Serial Number
By clicking on Serial Number once, the prompt should change to display your UDID
Since your UDID will never change (on its own), copy and paste it elsewhere for quick and easy access later in the future!
