The MMSI is the unique fingerprint of every AIS-equipped vessel. Here's how to read one.
A Maritime Mobile Service Identity (MMSI) is a 9-digit number that uniquely identifies a vessel's radio equipment. If AIS is the vessel's voice, the MMSI is its name.
The first three digits are the Maritime Identification Digits (MID) — a country code. 338 and 366–369 are the United States; 316 is Canada. That's how our API derives the flag shown on every vessel profile.
An MMSI can change if a vessel is re-registered or its radio is replaced. An IMO number is permanent for the hull's lifetime. For live tracking you almost always key on MMSI; for long-term identity, cross-reference the IMO.
Drop any 9-digit MMSI into our free MMSI lookup tool to see the last known position, or hit the API directly. No key required for the public lookup.