ITU callsign prefix - Country Codes
ITU call sign prefixes are allocated for radio stations, television stations, and aircraft registration identifiers by The International Telecommunication Union (ITU). For the most part, these country codes are agreed upon internationally. But a few countries, such as Australia and Canada, do not fully comply with these rules.
ITU call signs can be any combination of letters and numbers. Most countries have ranges of call signs that are allocated to them. For example, the United States is assigned AAA-ALZ, KAA-KZZ, NAA-NZZ, and WAA-WZZ.