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• ICAO 24-bit address

  • All modern aircraft are assigned a unique ICAO 24-bit address (also known as Mode-S 'hex code') since 2007 based on registration allocated by the relevant civil aviation authority.
  • Normally the address is never changed; however the onboard transponders can be wrongly programmed. They can also be swapped between aircraft with the maintenance team forgetting to reprogram the transponders.
  • There are 16,777,214 unique addresses available (2^24-2). Each address can be displayed in any of 3 formats: hexadecimal, octal or binary.

• Aircraft de-registered prior to 2007 should not be expected to have a 24-bit address allocated.
  Country allocations for hex ranges are available here.

• Hex codes are available to Extended-level subscribers and above.

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