Maritime glossary · Maritime Mobile Service Identity
MMSI
The nine-digit identifier broadcast by a vessel’s AIS and used to route VHF radio calls.
Definition
The MMSI is the radio-equivalent of a phone number for the ship’s AIS transponder, EPIRB beacon, and DSC radio. The first three digits — the MID (Maritime Identification Digits) — encode the country of registry, which is why an MMSI changes when a vessel is reflagged. Unlike the IMO number, which stays with the hull for life, the MMSI is tied to the ship’s current flag and is reissued when the registration moves. Both numbers travel together in the AIS broadcast.
Worked example
An MMSI starting with "244" identifies a Dutch-flagged vessel. Switch the flag to Liberia and the MMSI changes to start with "636". The IMO number stays the same.
How Vessel Hunter uses MMSI
Vessel Hunter resolves MMSI changes against the IMO number so a reflagged ship still surfaces in the same dossier — no broken history, no duplicate records.
Related terms
- AISAutomatic Identification System
The VHF radio system every commercial vessel uses to broadcast its position, course, and identity.
- IMO NumberInternational Maritime Organization Number
The seven-digit identifier permanently assigned to a ship — never reused, never changed, even on resale or reflag.
- Flag State
The country under whose laws a ship is registered — sets the regulatory standard for the vessel.
The bigger picture
MMSI is one piece of the commercial maritime picture Vessel Hunter pulls together for shipyards, port agents, and service providers. Every vessel record bundles AIS, ownership, inspections, dry-dock history, casualty record, classification status, and a verified contact for the operator decision-maker behind the ship — so the team that reaches out first wins the work.
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