Maritime Mobile Service Identity

A Maritime Mobile Service Identity (MMSI) is effectively a maritime object's international maritime telephone number, a temporarily assigned UID issued by that object's current flag state (unlike an IMO number, which is a permanent global UID).

An MMSI comprises a series of nine digits, consisting of three Maritime Identification Digits (country-codes), concatenated with a specific identifier. Whenever an object is re-flagged, a new MMSI must be assigned.

A "maritime object" could be anything that requests an MMSI identifier.—e.g., a vessel, fixed offshore installation, mobile unit, maritime aircraft, coast station, etc. Communications may be routed to "individual objects" or to "groups of objects". A group call to objects can be based on an object's locale, owner/operator/fleet, type, etc. or combinations thereof.

MMSI are formed in such a way that the identity or part thereof can be used by telephone and telex subscribers connected to the general telecommunications network to call any of these objects automatically. Communications are sent in digital form over a radio frequency channel.

As of 2024, there are six kinds of maritime mobile service identities:

An MID consists of three digits, always starting with a number from 2 to 7 (assigned regionally). A second MID can be assigned once the first or subsequently allocated MID is more than 80% exhausted, and the rate of assignments is such that 90% exhaustion is foreseen. A listing of MIDs assigned to each country can be found in Table 1 of ITU Radio Regulations Appendix 43.

The initial digits of an MMSI categorize the identity, as defined in by Recommendation M.585. The meaning of the first digit is:

The 9-digit code constituting a ship station identity is formed as follows: