Vessel Identification Guide
MMSI Numbers Explained
Understanding Maritime Mobile Service Identity numbers — the 9-digit codes that uniquely identify vessels and maritime stations in AIS.
Example MMSI
What is an MMSI Number?
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.
MMSI Number Structure
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.
992111234Maritime Identification Digits (MID)
The first 3 digits of a ship MMSI indicate the flag state. Here are common MID codes by region.
Europe
Americas
Asia
Africa & Oceania
This is a partial list. The ITU maintains the complete MID database.
MMSI vs IMO Number
Both identifiers are important for vessel tracking, but they serve different purposes.
MMSI Number
- 9 digits
- Assigned to radio equipment
- Can change if vessel reflagged
- Used in AIS position reports
- Indicates flag state via MID
IMO Number
- 7 digits (prefixed with "IMO")
- Assigned to ship's hull
- Never changes throughout vessel life
- Used in AIS static data (Type 5)
- Best for tracking vessel history
Best Practice
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.
Common MMSI Issues
MMSI data quality issues you may encounter and how to handle them.
Duplicate MMSIs
Multiple vessels may incorrectly use the same MMSI. This is a data quality issue, not a system error. Use IMO numbers to disambiguate.
Invalid MID Codes
Some vessels broadcast MMSIs with invalid or non-existent MID codes. This may indicate misconfiguration or attempts to obscure identity.
MMSI Changes
When a vessel changes flag, it receives a new MMSI. Historical data may show the same vessel with different MMSIs over time.
Test/Default MMSIs
MMSIs like 000000000 or 123456789 are test values that should not be used in production. Filter these out in your application.
Query Vessels by MMSI
Use our API to look up vessel positions, details, and history using MMSI or IMO numbers.