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ISO image (disc image file)

An ISO image is a single-file representation of an optical disc's full contents and file system, used for burning, mounting, distributing software and creating bootable media across operating systems.

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Overview

An ISO image is a disk image stored as a single file that contains the complete contents and file system structure of an optical disc such as a CD, DVD or Blu-ray. It preserves the arrangement of files, directory metadata and sector layout so the image can be written to another disc or mounted as a virtual drive without loss of information. The most common filename extension is .iso.

Structure and standards

ISO images typically implement the ISO 9660 standard for the on-disc file system; extensions and alternatives include Joliet (Microsoft), Rock Ridge (UNIX-style metadata) and UDF (used for DVDs and Blu-ray). Bootable ISO images follow the El Torito specification so a computer can start from the image as if from a physical bootable disc.

Common uses

  • Software distribution: operating system installers and large application suites are commonly distributed as ISO files.
  • Burning and duplication: images can be written to blank optical media to produce identical discs.
  • Virtual mounting: modern operating systems and virtualization platforms mount ISOs as virtual optical drives for installation or file access without burning.
  • Archiving: images preserve an exact snapshot of a disc for long-term storage or forensic analysis.

Creation and access

ISO images are created by disc authoring and imaging tools that read a physical disc or compile files into a disc image. They can be checked with checksums to verify integrity. Operating systems often provide built-in support to mount ISO files; third-party utilities and virtualization software add extra features such as writable virtual drives and conversion to other image formats.

Variants and distinctions

Not all disc image files are identical: formats like .bin/.cue, .img or proprietary container types differ in header information, track layout or copy-protection data. A hybrid ISO supports both optical and bootable USB use. Compressed forms (.iso.gz) reduce storage size but require decompression before use or on-the-fly handling by tools.

History and notable facts

The term "ISO" derives from the ISO 9660 standard, developed for interoperability of optical media. ISO images remain a convenient, platform-independent method for distributing large software packages and for creating reproducible installation media, despite the declining use of physical optical discs.

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