Once you have applied and received your Carte Vitale from CPAM (French Social Security) you might wonder how they allocated your social security number.
If you wonder how to obtain a Carte Vitale from the French health system, see my post dated 01.02.2010 - Registering with the French Health System (CPAM, RAM, Mutuelle Action, etc).
The Carte Vitale has 15 digits.
The first one is either 1 for male or 2 for female. Then xx is your year of birth, xx your month of birth, xx is the department where you were born (or 99 is you were born outside France), xxx is the code of the city where you were born, xxx is the number according to the month and the commune where you were born, and the last tow xx are allocated by the social security for control purpose.