Glossary · Equipment & tracking
IMO number, MMSI and call sign: how to identify a vessel
Also known as IMO number, MMSI, and Call sign.
Ships change name, flag and owner. Only one identifier survives that, which matters when you are confirming the vessel carrying your fruit.
Published
Your booking names a vessel. Six weeks later a reefer of grapes is somewhere at sea and you want to confirm it is on the ship you think it is — except the vessel was substituted, or your box was lifted off at a hub onto something else entirely, or you searched the name and found three ships called it.
Ships change name, flag, owner and operator, sometimes several times in a decade. Only one of their identifiers survives all of that, and knowing which is the difference between confirming your cargo and confidently watching the wrong ship.
The four identifiers
| Identifier | Format | Assigned by | Changes when? |
|---|---|---|---|
| IMO number | IMO 9074729 — 7 digits |
IMO’s registry | Never. Fixed to the hull for life |
| MMSI | 9 digits, e.g. 477123456 |
The flag state | On reflagging |
| Call sign | Alphanumeric, e.g. VRAB7 |
The flag state | On reflagging |
| Vessel name | Free text | The owner | Freely — sale, charter, rebrand |
IMO number — the only permanent one
Seven digits, issued once when the hull is built and never reassigned. It survives sale, renaming, reflagging and rebuilding, and it is retired with the ship when she is scrapped.
The last digit is a check digit, so a mistyped IMO number can be detected rather than silently looking up a different vessel.
If you record one identifier, record this one.
MMSI — the one AIS actually broadcasts
Nine digits, assigned by the country of registry. The first three are the Maritime Identification Digits, which encode the flag: 477 is Hong Kong, 636 is Liberia, 538 is the Marshall Islands.
This is the number AIS transmits and the primary key most tracking systems index on — which creates a trap. MMSI changes when a vessel reflags, and the old number can later be assigned to a different ship. Historic data keyed on an MMSI is not guaranteed to be about one vessel.
Call sign — radio, mostly
Assigned by the flag state for radio identification. It appears on documents and in AIS static data, and it changes on reflagging like the MMSI. Rarely the most useful thing to hold.
Vessel name — the least reliable
Names change on sale or charter, and they are reused. Several ships have shared a name over the years, and some names are common enough that a search returns a handful of candidates.
Searching a name is how people end up tracking the wrong ship. If a name is all you have, cross-check against the IMO number before you believe anything.
What this means when a reefer is at sea
The vessel on your booking may not be the vessel that sails. Substitutions happen, and the paperwork does not always catch up.
After a transshipment your container is on a different ship. The vessel named on your original B/L completes its rotation and arrives on time, reporting perfectly, with your fruit on a quay behind it. Watching that vessel tells you nothing.
So the vessel is not the thing to track. Carrier milestones against your bill of lading or container reference follow the cargo. The vessel identifiers are for confirming which ship a milestone refers to — genuinely useful, but a second-order question.
Where to find them
The bill of lading and the arrival notice normally carry the vessel name and voyage number. IMO and MMSI are less consistently printed — the carrier’s own schedule pages list them, and any AIS-based service will show all four together once you have any one of them. Start from the IMO number if you can.
Verify your vessel in 30 seconds
Worth doing before you believe any position, and it takes less time than reading this paragraph.
Take the vessel name off your paperwork and search it on a public AIS site — MarineTraffic, VesselFinder, any of them. If more than one ship comes back, that is the problem this page exists for. Open the candidate and check the IMO number against your documents: it matches or it doesn’t, and no amount of matching names, flags or plausible routes substitutes for that one field.
Then glance at the flag. If it disagrees with what your paperwork says, you have either the wrong vessel or a ship that reflagged — in which case the MMSI and call sign on your documents are out of date and the IMO number is still right.
Thirty seconds, and you know whether the map you are about to trust is showing your cargo’s ship or a different one with the same name.
FAQ
What is the difference between an IMO number and an MMSI?
The IMO number is permanent — fixed to the hull for its whole life, surviving sale, renaming and reflagging. The MMSI is assigned by the flag state and changes when the ship reflags, and can later be reassigned to a different vessel. AIS broadcasts the MMSI; the IMO number is what makes an identification durable.
Can two ships have the same name?
Yes, both at the same time and across the years, and common names recur. This is the main reason searching by name returns the wrong vessel. Confirm against the IMO number.
Does the IMO number change if the ship is sold or renamed?
No. That is the entire point of it. It changes for nothing, and is retired only when the vessel is scrapped.
Should I track the vessel or the container?
The container — or better, the bill of lading. Vessel tracking follows a ship; after a transshipment or a vessel substitution, that ship is no longer carrying your cargo. Use the identifiers to confirm which vessel a carrier milestone refers to, not as your primary tracking key.