Apple has closed the signing window for iOS 8.1 on compatible iPhone, iPad and iPod touch models, thereby eliminating the ability for users to downgrade to the software version. iOS 8.1.1, seeded as an over-the-air update a few weeks ago, is now the earliest software version that devices can be restored to.
If you are currently using an iPhone, iPad or iPod touch that is running iOS 8.1, keep in mind that you will be unable to ever go back to that software version if you upgrade at this point. The good news for jailbroken users is that an iOS 8.1.1 untethered jailbreak has already been released for Windows, with an OS X tool on the horizon.
iOS 8.1.1 is compatible with the iPhone 4s, iPhone 5, iPhone 5c, iPhone 5s, iPhone 6, iPhone 6 Plus, iPad 2, iPad 3rd generation, iPad 4th generation, iPad Air, iPad Air 2, iPad mini, iPad mini 2, iPad mini 3 and iPod touch 5th generation. iOS devices purchased from Apple Stores should be running iOS 8.1 – iOS 8.1.1 out of the box.