INSEE code
The official code identifying a French commune, department or region in the Code Officiel Géographique kept by INSEE.
The INSEE code is the primary key of the Code Officiel Géographique, the register of French territorial units maintained by the Institut national de la statistique et des études économiques. A unit keeps its code for good.
That is what makes the code more reliable than the name: 1,441 of the 34,969 commune names are shared by two or more communes. Apremont alone exists six times.
The length states the tier. A commune has five characters (28001), and Corsican and overseas codes may contain a letter, as in 2A004. A department has two or three (01, 2A, 971), a region two digits (11).
For the region tier outside France, use the ISO 3166-2 code.