The ENI number or European Number of Identification is a registration number for vessels on inland European waters. Since April 2007, it has been required for vessels longer than 20 meters or more than 100 cubic meters. Tugs, push boats, service vessels and passenger vessels require also an ENI number. The number is unique and attached permanently to the vessel for its entire lifetime. You find this number on each side and stern of the vessel.
It is a eight digit identifier. The first 3 digits identify the competent authority and the other digits are a serial number.