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, which is a global forever UID). An MMSI comprises a series of nine digits, consisting 3-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 which 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. Types. There are now six kinds of maritime mobile service identities: Maritime identification digits (MID). 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 first digit of an MMSI. The initial digits of an MMSI categorize the identity, as defined in by Recommendation M.585. The meaning of the first digit is: Station Identity Formats. Ship station identities. The 9-digit code constituting a ship station identity  is formed as follows: where MID represent the Maritime Identification Digits and X is any figure from 0 to 9. If the ship is fitted with an Inmarsat B, C or M ship earth station, or it is expected to be so equipped in the foreseeable future, then the identity should have three trailing zeros: If the ship is fitted with an Inmarsat C ship earth station, or it is expected to be so equipped in the foreseeable future, then the identity could have one trailing zero: If the ship is fitted with an Inmarsat A ship earth station, or  has satellite equipment other than Inmarsat, then the identity needs no trailing zero. Group ship station call identities. Group ship station call identities for calling simultaneously more than one ship are formed as follows: where the first figure is zero, and X is any figure from 0 to 9. The particular MID represents only the country assigning the group ship station call identity and so does not prevent group calls to fleets containing more than one ship nationality. Coast station identities. Coast station identities are formed as follows: where the first two figures are zeros, and X is any figure  from 0 to 9. The MID reflects the country in which the coast station or coast earth station is located. Group coast station call identities. Group coast station call identities for calling simultaneously more than one coast station have the same format as individual coast station IDs: two leading zeros, the MID, and the four digits. They are formed as a subset of coast station identities, as follows: US Coast Guard stations use a non-standard MMSI: 003669999 - any US Coast Guard Base station Note that administrations in other countries may use different formats. Reference: ITU-R Recommendations M.585-7  Search and Rescue Transmitter. AIS Search and Rescue Transmitters (AIS-SART) have an identifier related to the manufacturer, rather than a country's MID: The digitals represented by the two Y characters are assigned by the International Association for Marine Electronics Companies and refer to the SART manufacturer, while the Xs are sequential digits assigned by the manufacturer identifying the SART. Federal US MMSIs. In the United States (one of whose MIDs is 366), federal MMSIs are assigned by the National Telecommunications and Information Administration, and are normally (but not always) formed as 3669xxxxx. Non-federal MMSIs are assigned by  the Federal Communications Commission normally as part of the ship station license application, and are formed as 366xxx000 for ships on international voyages and ships needing an Inmarsat mobile earth station, or 366xxxxx0 for all other ships. The United States Coast Guard group ship station call identity is 036699999, and group coast station call identity is 003669999. In the U.S., MMSIs are primarily used for digital selective calling and for assigning Inmarsat identities. Exhaustion of MMSIs. Because all ships on international voyages, as well as all ships fitted with an Inmarsat B or M ship earth station, are assigned MMSIs  of the format MIDxxx000, a serious problem has arisen internationally in assigning sufficient numbers of MIDs to all administrations that need them. For example, a country having 10,000 Inmarsat-equipped ships would require 10 MIDs just to accommodate those 10,000 ships. If 50,000 boaters decided to fit small Inmarsat M terminals, 50 additional MIDs would be required to accommodate them. The problem exists with Inmarsat-equipped ships because ITU-T recommendations require that Inmarsat ship earth stations be assigned the identity (MESIN) TMIDxxxYY, where T indicates the type of Inmarsat station, YY indicates the Inmarsat station extension (e.g."00" might  indicate a telephone in the bridge, "01" might indicate a fax machine in the radio room, etc.), and MIDxxx indicates the ship station number, which relates to the assigned ship station identity MIDxxx000. The MMSI was meant to be an all-inclusive ship electronic identity, used in one form or another by every GMDSS or telecommunications instrument on the ship. Questions have been raised, however, whether the MMSI can in practice totally fulfill that role. ITU may eventually end the practice of relating Inmarsat MESIN identities with the ship MMSI identity. To help mitigate against MMSI number exhaustion, manufacturers are  required to cripple DSC-capable radios so that an MMSI number can only be entered once, which means that a device owner cannot move the radio from one boat to another. This is especially egregious for hand-held VHF devices, for which a single MMSI number makes little sense. The World Radio Conference, Geneva, 1997 (WRC-97), adopted Resolution 344 concerning the exhaustion of the maritime mobile service identity resource. In view of improvements to public switched telephone networks, and new capabilities of the Inmarsat system other than Inmarsat B or M, previous restrictions should no longer be applicable. All  nine digits of the MMSI can be used in such cases, and no longer need to end in trailing zeros.