A lot of us have been rather hasty to jump the iOS 6 bandwagon and upgrade to this latest firmware. Most users did so primarily to get a taste of the new release and lay their hands on some of the awesome features that Apple has included in iOS 6. However, the problem is that if you wish to revert back to a lower firmware from iOS, you will run into a whole host of problems.
One of the main problems that you will face when downgrading your iOS devices from iOS 6 to, say, iOS 5.1.1 is that you will face an error 1600. This problem occurs when iTunes is unable to verify from an Apple server that your iOS device is eligible for the firmware it is asking for.
The whole thing occurs because when you are running Cydia and then try to restore your device to an earlier firmware, a host entry redirects gs.apple.com to a Cydia server rather than an Apple server. Naturally, when iTunes try to verify your device, this data is sent over to a Cydia server which doesn’t generate an appropriate response and as a result, iTunes ‘thinks’ that your device is ineligible for the firmware.
Here’s an easily and convenient solution to counter this problem. You can simply install the tool ‘TinyUmbrella.’ With the help of this tool, you can decide the data routed through Cydia. Launch the tool and simply uncheck the ‘Set hosts to Cydia on Exit’.
After doing so, launch iTunes again and try to restore to your older firmware. This time, you won’t face the error 1600.
Source: Jailbreak Matrix