Insights · Ship identity · IMO vs MMSI
They look similar — a string of digits attached to a ship — but they answer different questions. The IMO number names the hull for life; the MMSI names the radio station and changes the moment the ship swaps flags. Knowing which is which is the difference between tracking a vessel and losing it.
A ship at sea is constantly announcing who it is — but it does so with more than one identity, and the two most important are easy to confuse. The IMO number and the MMSI are both numeric, both tied to a single vessel, and both turn up in maritime databases side by side. Yet they belong to completely different systems and behave in completely different ways. Get them mixed up and you can end up tracking the wrong ship entirely.
The short version: the IMO number identifies the hull, and the MMSI identifies the radio station bolted to it. One is permanent; the other is not.
An IMO number is a unique seven-digit figure assigned to a ship under the IMO Ship Identification Number Scheme — introduced by resolution A.600(15) in 1987 and made mandatory under SOLAS chapter XI-1. It is allocated when the hull is built and stays with that hull until it is scrapped. Sell the ship, rename it, move it to a new flag, change its operator — the IMO number does not move. That permanence is the whole point, and it is why it is the reliable anchor for any historical record. We cover the format and its check digit in detail in what is an IMO number.
The MMSI — Maritime Mobile Service Identity — is a nine-digit number assigned by a ship's flag administration and programmed into its radio equipment. It is the identity used by the Automatic Identification System (AIS), by Digital Selective Calling (DSC) on VHF and MF/HF radios, and by maritime distress and safety calling. In effect it is the "phone number" of the vessel's transponder.
Crucially, the first three digits of an MMSI are the Maritime Identification Digits (MID), a code that identifies the country of registration. A vessel flagged in the Netherlands carries an MMSI beginning 244 or 245; a Panama-flagged ship begins 351–357; a Marshall Islands ship begins 538, and so on. Because the country is baked into the number itself, the MMSI changes whenever a ship reflags — the old administration's MMSI is surrendered and a new one is issued under the new MID. The same physical hull can therefore wear several MMSIs across its career.
| Property | IMO number | MMSI |
|---|---|---|
| Length | 7 digits | 9 digits |
| Identifies | The hull | The radio station |
| Assigned by | IMO scheme (via S&P Global) | Flag administration |
| Encodes flag? | No | Yes — first 3 digits (MID) |
| Changes on reflag? | Never | Yes |
| Used by | Registries, certificates, screening | AIS, DSC, VHF/MF/HF radio |
This is where the two identities meet in the real world. The AIS transmitter aboard a ship sends out a stream of messages, and the IMO number and MMSI live in different parts of that stream:
That difference in frequency matters. A receiver picks up an MMSI within seconds, but may have to wait minutes for the static message that carries the IMO number. To read more about how these messages turn into a track on a map, see what is AIS ship tracking.
Because the MMSI is programmable, surrendered on reflagging, and only loosely policed, it is the weaker of the two for identification. MMSIs get recycled when administrations reuse retired numbers, mistyped when equipment is reprogrammed, and deliberately spoofed by vessels that want to appear as something they are not. A ship can broadcast an MMSI that points to one identity while its hull-stamped IMO number tells the truth.
The MMSI tells you what a ship is saying about itself. The IMO number tells you what the ship is. When the two disagree, believe the hull. ITU Radio Regulations, Appendix 43 (MID allocation) & IMO resolution A.1117(30)
This is why every vessel file in the Marifest registry is keyed to the IMO number, with the MMSI recorded as a secondary, mutable attribute. When a tracked position on the live map arrives by MMSI, Marifest resolves it back to the permanent IMO-anchored record, so a reflagging or a recycled MMSI never quietly splits one ship into two.
For completeness, ships carry a third identifier: the radio call sign, a short alphanumeric string (for example PBDC) also assigned by the flag state and tied to the radio licence. Like the MMSI, the call sign is allocated by the administration and changes on reflagging; it too is broadcast in the AIS static message. It is useful for voice radio but, like the MMSI, it is not a permanent identity. For long-term tracking and compliance, the IMO number remains the only fixed point.
How Marifest uses it
Marifest reads both identifiers from the AIS feed and reconciles them against the permanent hull record, so a position broadcast by a mutable MMSI always lands on the right vessel file.
Live AIS reports arrive keyed by MMSI; Marifest resolves each one to the IMO-anchored file, so a reflagging never breaks a ship's track history.
The first three digits of the MMSI reveal the broadcasting flag state, which Marifest cross-checks against the registered flag on the hull record.
When a broadcast MMSI or call sign disagrees with the permanent identity, the mismatch is surfaced rather than silently trusted.
Old and new MMSIs, names and call signs all attach to the single IMO number, keeping every ship as one continuous record.