Vessel Identification Guide
Understanding Maritime Mobile Service Identity numbers — the 9-digit codes that uniquely identify vessels and maritime stations in AIS.
Example MMSI
MMSI (Maritime Mobile Service Identity) is a unique 9-digit number used in maritime communications to identify vessels, coast stations, and other maritime entities. It's the primary identifier used in AIS (Automatic Identification System) transmissions.
Unlike the IMO number which is permanently assigned to a ship's hull, the MMSI is assigned to the ship's radio equipment. This means an MMSI can change if a vessel changes flag state or replaces its radio equipment.
The first three digits of a ship station MMSI form the Maritime Identification Digits (MID), which indicate the country where the vessel is registered. This allows you to quickly identify the flag state of any vessel from its MMSI.
The format of an MMSI varies depending on the type of station it identifies.
Standard vessel MMSI. First 3 digits are the MID (Maritime Identification Digit) indicating flag state.
211234567Shore-based radio stations. First two digits are 00, followed by MID.
002111234For addressing a group of ships. First digit is 0, followed by MID.
021123456Search and rescue aircraft. Starts with 111, followed by MID.
111211123Search and Rescue Transponders. Always starts with 970.
970012345Man Overboard devices. Always starts with 972.
972123456Emergency Position Indicating Radio Beacons with AIS. Starts with 974.
974123456Auxiliary craft associated with a parent ship. Starts with 98.
982111234Navigation aids like buoys and lighthouses. Starts with 99.
992111234The first 3 digits of a ship MMSI indicate the flag state. Here are common MID codes by region.
This is a partial list. The ITU maintains the complete MID database.
Both identifiers are important for vessel tracking, but they serve different purposes.
Use IMO numbers when you need to track a vessel over time or across ownership changes. Use MMSI for real-time tracking and current position lookups. Our API supports queries by both identifiers.
MMSI data quality issues you may encounter and how to handle them.
Multiple vessels may incorrectly use the same MMSI. This is a data quality issue, not a system error. Use IMO numbers to disambiguate.
Some vessels broadcast MMSIs with invalid or non-existent MID codes. This may indicate misconfiguration or attempts to obscure identity.
When a vessel changes flag, it receives a new MMSI. Historical data may show the same vessel with different MMSIs over time.
MMSIs like 000000000 or 123456789 are test values that should not be used in production. Filter these out in your application.
Use our API to look up vessel positions, details, and history using MMSI or IMO numbers.