iREB a small handy utility for Windows and Mac that can solve the mistakes “16xx” and “21” from iTunes that are displayed after a reset or a firmware downgrade. iREB is developed by iH8sn0w, and today he released a new update for iREB. The application is only compatible with EDGE iPhone, iPhone 3G and iPod Touch 1G and can be downloaded from here. The tool is available for both Windows and Mac operating system, we advised you to save this program on a secure place on your hard drive because it’s a one of it’s kind utility.
When you will be presented such an error will not have to do is start and follow the instructions on the screen. I recommend you download it and keep it somewhere in your computer .. can come in useful