Vessels can be identified using several different identifiers. The most common ones used in AIS data and vessel tracking systems are IMO number, MMSI, and vessel name.

These identifiers serve different purposes and have different levels of stability over time. Understanding their differences helps when querying vessel data or interpreting tracking results.

Overview

IMO number

The IMO number (International Maritime Organization number) is the permanent identifier assigned to a vessel for its entire lifetime.

It is typically a seven-digit number, for example: 9786815.

It uniquely identifies the physical vessel, remains the same even if the vessel changes owner, flag, or name, and does not change during the vessel's operational life.

Because of this stability, the IMO number is the most reliable identifier for tracking a vessel over time.

In most APIs and maritime datasets, the IMO number is the preferred way to query vessel information.

MMSI

The MMSI (Maritime Mobile Service Identity) identifies the AIS transponder installed on the vessel.

It is a nine-digit number used in radio communication and AIS transmissions, for example: 563318600.

Unlike the IMO number, the MMSI is not permanently tied to the vessel. It may change when the vessel changes flag state, when the AIS transponder is reconfigured or replaced, or when administrative updates occur.

Because of this, the MMSI should generally be considered an operational identifier, rather than a permanent vessel identifier.

Vessel name

The vessel name is the human-readable name assigned to the ship. For example: EVER GENIUS.

Vessel names can change multiple times during a ship's lifetime due to ownership changes, charter agreements, or company rebranding. As a result, vessel names are not guaranteed to remain stable over time.

That's why we like to keep track of previous names (often referred to as former names or FKA – formerly known as).

Identifier changes and trajectory continuity

When reconstructing vessel trajectories, Searoutes handles these identifier changes by relying on the IMO number, which represents the physical vessel and remains stable throughout its lifetime.

As a result, when querying a vessel trace over a time range where identifiers such as MMSI have changed, the trajectory remains continuous. AIS positions associated with different MMSI values can still be linked to the same vessel through their shared IMO number. This ensures that historical trajectories are reconstructed consistently even when operational identifiers evolve over time.

Summary

When querying vessel data, the choice of identifier depends on the level of reliability required. In most cases:

• IMO should be used when a stable identifier is required.
• MMSI can be used for AIS-related operations.
• Vessel name can be used for human-readable searches, but may be ambiguous or change over time.

For historical tracking and long-term vessel identification, the IMO number is the most reliable identifier.