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The United States Coast Guard (USCG) Navigation Center (NavCen) has made available a list of blocks of MMSI numbers and which MMSI registering agency has been allocated those blocks. This information can be useful if a ship has already been assigned an MMSI but the registrant of the MMSI and which agency has registered the boat are unknown.

List of MMSI Block Allocations
https://www.navcen.uscg.gov/sites/default/files/pdf/MMSIAllotments.pdf