Aircraft Registration Marks, I
The registration you will find on an aircraft is a combination of a country code and a country registration number (or letters). The country codes are assigned by the International Telecommunication Union (ITU), this is because they are also used as radio call signs and that part is governed by the ITU.
The aircraft nationality marks are requested at the ITU from a list assigned to that state and then reported to ICAO, and we have a copy of the latest list here below.
Nationality and Registration Marks
Aircraft nationality marks as notified to ICAO, Nationality marks arranged alphabetically by State