When it comes to downloading copies of software from the Internet, it is common practice for the software to be distributed in an archived form. This not only means that every part of the software is downloaded, and therefore there is no need to go hunting for missing files, but it also ensures that the files will not have been tampered with unless special archive-editing software is used. However, the different file formats can be confusing, so it is important to know the differences between ISO and IMG files if they are to be handled correctly once downloaded.
ISO and IMG are both archival formats. Each file contains a copy of the contents of the original disc the archive was made from, plus information about the file structure of the disc. They are designed to make archiving the disc easier and make creating an exact duplicate copy easier. However, while there is only one version of the ISO format, IMG comes in two versions: compressed and uncompressed.
ISO File Format
The ISO file format is an international standard file format designed to contain a disc image, including its file system, in a single file. The ISO format was created as a means of archiving discs and as such it has become a standard method of distributing software, such as Linux installation CDs, for users to download and burn their own CDs. ISO is used to create CD and DVD images.
IMG File Format
The IMG file format was designed to create a backup copy of a floppy disk in a single file. It works by creating a bitmap of each sector of the disk that has been written to. As these sectors are 512 bytes in size, IMG files are always sized in multiples of 512 bytes. Since the demise of floppy disks, the IMG format has been used for the creation of hard disk image files.
There is no difference in the structure of ISO and IMG formats if the IMG file is uncompressed. It is possible for an IMG format file to be renamed with the ISO file extension and then opened in software that only recognises the ISO file format. This is an effective way of accessing disc information in programs that do not handle the IMG format.