MMSI

MMSI stands for Maritime Mobile Service Identity. It is a nine-digit numeric identifier assigned to ships, coast stations, and certain service groups to identify transmitting stations in maritime communications. The MMSI is used by systems such as digital selective calling (DSC), the Automatic Identification System (AIS), VHF radio, satellite communications, and emergency signaling to route messages and calls.

Structure and allocation: Each MMSI consists of nine digits. The first three digits are Maritime Identification Digits (MIDs), allocated by the ITU to a country or region. The next three digits identify the vessel, group, or coast station within that MID, as assigned by the national administration. The final three digits provide a unique identity within that subset, enabling distinct addresses for ships or stations within the same MID.

Categories and usage: Ship station MMSIs identify individual vessels, coast station MMSIs correspond to shore-based facilities, and group MMSIs are used for coordinated calls to a fleet or area. Some special MMSIs are reserved for search and rescue, coast guard, or other services. In practice, MMSIs accompany AIS messages, DSC calls, and other digital protocols to ensure precise routing and addressing.

Administration and regulation: ITU allocates MIDs to countries; national administrations issue and manage the remaining digits to vessel owners and operators. MMSI numbers must be unique within each MID and are managed to prevent duplication. They enable automated, globally understood addressing across maritime communication networks.