<P> In the United States , a Selective Service Number must be applied for by all male citizens turning age 18 . An optional national identity number is the Social Security number ( SSN ) , a nine - digit number issued to U.S. citizens , permanent residents , and temporary ( working ) residents . Its purpose was to identify individuals for the purposes of Social Security , but it is now also used to track individuals for taxation purposes . There is no legal requirement to have a SSN if it is not required for Social Security or taxation purposes , but in practice one is required for many other purposes , for example to open a bank account or apply for a driving license , so that nearly all U.S. citizens and permanent residents have one . The SSN has therefore become a de facto national identification number , despite the fact that originally it was expressly not for this purpose . In fact , a valid SSN can be easily guessed , as they were issued serially prior to June 25 , 2011 . </P>
<P> In the United States , a Selective Service Number must be applied for by all male citizens turning age 18 . An optional national identity number is the Social Security number ( SSN ) , a nine - digit number issued to U.S. citizens , permanent residents , and temporary ( working ) residents . Its purpose was to identify individuals for the purposes of Social Security , but it is now also used to track individuals for taxation purposes . There is no legal requirement to have a SSN if it is not required for Social Security or taxation purposes , but in practice one is required for many other purposes , for example to open a bank account or apply for a driving license , so that nearly all U.S. citizens and permanent residents have one . The SSN has therefore become a de facto national identification number , despite the fact that originally it was expressly not for this purpose . In fact , a valid SSN can be easily guessed , as they were issued serially prior to June 25 , 2011 . </P>
how many digits is a social security number
how many digits is a social security number
nine
nine