An invalid (or impossible) Social Security number (SSN) is one which has not yet been assigned.

The SSN is divided as follows: the area number (first three digits), group number (fourth and fifth digits), and serial number (last four digits).

To determine if an SSN is invalid consider the following: No SSNs with an area number in the 800 or 900 series, or "000" area number, have been assigned. No SSNs with an area number above 772 have been assigned in the 700 series.

No SSNs with a "00" group number or "0000" serial number have been assigned. No SSNs with an area number of "666" have been or will be assigned. Information about the SSN and SSNs that have been assigned is available at High Group List and Other Ways to Determine if an SSN is Valid .