Create A .IPSW File Using The .DMG Files Extracted From A .ZIP Direct iPhone Firmware Download
If you are in process of downgrading your iPhone or iPod Touch you will more than likely be downloading the older firmware directly from apple.com. When you download this firmware it is provided as a zip file with a .zip file ending such as iPhone2,1_3.1_7C144_Restore.zip. If you unpack this file it will include a couple directories and numerous files as listed below.