dal manuale mysql...
Note: There can be only one AUTO_INCREMENT column per table, it must be indexed, and it cannot have a DEFAULT value. As of MySQL 3.23, an AUTO_INCREMENT column works properly only if it contains only positive values. Inserting a negative number is regarded as inserting a very large positive number. This is done to avoid precision problems when numbers ``wrap'' over from positive to negative and also to ensure that you don't accidentally get an AUTO_INCREMENT column that contains 0.
Quel numerazzo e' il max value per un INT SIGNED.