Files that contain the .ipsw file suffix normally hold update and restore information that is used by the Apple iTunes software application.
The information that is contained within the IPSW files is used by iTunes for iPhone and iPod updates and software restores. When a user downloads new music or restores information that has been lost by a device, the IPSW file will be used to update or restore the user's iPod or iPhone.
The IPSW file is actually a renamed zip archive. The data in the IPSW files is encrypted and can be installed using the Apple iTunes software.
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