While following the device registration process, the user has to enter the UDID number of the device.

To find the UDID of a device

1. Connect the iPhone to a computer using USB, and then launch iTunes.
   The page appears as shown in the following figure:
   
2. In the upper-left corner, select the required iPhone.
3. On the left navigation menu, click Summary.
4. In the right panel, click the serial number of the device to display the UDID.
   The page appears as shown in the following figure:
   
5. From the right panel, copy the UDID number to use it while device registration.