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Three distinct vessel identifiers, easily confused: IMO Number (permanent hull identifier), MMSI (radio identifier), and Call Sign (radio identifier). What each is for + how they interact.
| Criterion | IMO Number Permanent hull identifier. | MMSI Radio identifier (AIS + DSC). | Call Sign Radio voice-comm identifier. |
|---|---|---|---|
| Format | 7 digits | 9 digits (first 3 = flag MID code) | Alphanumeric, 4-7 characters |
| Assigned by | IHS Markit on behalf of IMO | Flag state administration | Flag state administration |
| Changes when... | Never (permanent for hull life) | Flag changes | Flag changes |
| Used for | Regulatory identification, ownership tracking, insurance | AIS transmission, DSC distress alerts | VHF voice communication |
| Mandatory since | 1996 (SOLAS XI-1/3) | 1999 (SOLAS IV/6) | Radio Regulations 1908 |
| Public lookup | Equasis, MarineTraffic | MarineTraffic AIS overlay | ITU MARS database |
Use for regulatory + ownership + insurance + P&I claims + PSC records. The permanent identifier.
Use for AIS lookup + DSC alerting + Sat-C messaging. The digital-radio identifier.
Use for VHF voice communication. Radio operator's identifier.
Last verified