Ships are often identified by one of two identifiers: their call sign (aka call name, call letters, call signal, or call) and their IMO number. A vessel may have one, both or neither of these – but the vessels on the OFAC SDN list will have both.

IMO numbers are unique numbers assigned to passenger ships of 100 gross tons or greater, and cargo ships of 300 gross tons or greater. It consists of the letters IMO followed by a seven digit number. The seventh digit is a check digit: if you multiply the first digit by 7, the second by 6, and so on, and add all the products together, the last digit of the result should match the check digit.

The IMO number, which is assigned by IHS Fairplay (formerly Lloyd’s Register-Fairplay) under the auspices of the International Maritime Organization (IMO), must be permanently marked on a visible part of the hull which contains the ship’s machinery, or its superstructure. For a passenger vessel, the marking must be on a horizontal surface visible from the air.

IMO numbers were first instituted in 1986, and became a mandatory feature in 1996. The requirement to have the IMO number permanently marked in a visible place was added in 2002.

IMO numbers, by the way, are also assigned to registered ship management companies. They follow the same structure (e.g. IMO 1234567).

The practical upshot of all this is that you can’t just take out a bucket of paint and rename your ship. You have to tell IHS Fairplay about it so the IMO number can be associated with the new vessel name. And that is how, when the Iranians changed the names of all their sanctioned IRISL (Islamic Republic of Iran Shipping Lines) vessels, the world knew what they were up to.

Call signs are not for the vessel, per se, but for the radio transmitter. The call signs are assigned by the national licensing authority for the country that the vessel is flagged under. In the US, commercial craft call signs start with either W or K (like commercial TV and radio stations), followed by a series of letters and numbers. The call signs for US naval vessels all start with the letter N. The exception to this rule is that leisure craft with a VHF radio transmitter, while they can apply for a radio license and a call sign, more often identify themselves by their boat’s name.

Links:

Call sign information for Ships and Vessels (Wikipedia)
IMO Number (Wikipedia)
IMO Number Search & Request