Usually registration as a prisoner or as a convict at the National Insurance Institute is done automatically, via information received from the Prison Service (except for those in house arrest). If, for some reason, the information was not received, a Form BL/6101 must be filled out and a certificate from the Prison Service, indicating your identifying information and the length of your imprisonment, must be enclosed in attachement.
Those in house arrest shall fill out a Form BL/6101 and attach a copy of the court protocol regarding the detention.