Every iPhone, iPod touch and iPad has a unique identifier number associated with it, known as a UDID (Unique Device ID). Your UDID is a 40-digit sequence of letters and numbers that looks like this: 0e83ff56a12a9cf0c7290cbb08ab6752181fb54b. It’s common for developers to ask for your UDID as they require it to give you beta copies of iOS apps.
Finding your UDID using iTunes
-Open iTunes (the Mac or PC app, not iTunes on your iPhone).
-Plug in your iPhone, iPod touch or iPad.
-Click its name under the devices list.
-Ensure you’re on the Summary tab.
-Click on the text that says Serial Number. It should change to say Identifier (UDID).
-Select Copy from the Edit menu.
-Your UDID is now in the clipboard, so you can paste it into an email or message.