erikthewise said:
If someone knows your last four digits, they know the only random part of it. The first three digits can be pretty much guessed by someone who knows where you were born and where you grew up. The middle two digits do not appear to be at all random either. That's why I hate to have the last four digits used for ID.
from wikipedia-----
The Social Security number is a nine-digit number in the format "111-11-1111." The number is divided into three parts.
The Area Number is assigned by the geographical region. Prior to 1975, cards were issued in local Social Security offices around the country and the Area Number represented the office code in which the card was issued. This did not necessarily have to be the in the area where the applicant lived, since a person could apply for their card in any Social Security office. Since 1972, when SSA began assigning SSNs and issuing cards centrally from Baltimore, the area number assigned has been based on the ZIP code in the mailing address provided on the application for the original Social Security card. The applicant's mailing address does not have to be the same as their place of residence. Thus, the Area Number does not necessarily represent the State of residence of the applicant, either prior to 1972 or since.
Generally, numbers were assigned beginning in the northeast and moving westward, so that people on the east coast had the lowest numbers and those on the west coast had the highest numbers. As the areas assigned to a locality are exhausted, new areas from the pool are assigned, so some states have noncontiguous groups of numbers.
Complete list of area number groups from the Social Security Administration
The middle two digits are the group number. They have no special geographic or data significance but merely serve to break the number into conveniently sized blocks for orderly issuance.
There is a theory that the two middle digits can be used to identify a person's ethnic background. This is debunked as an urban legend on snopes.com as well as on the Social Security Administration's website.
The group numbers range from 01 to 99. However, they are not assigned in consecutive order. For administrative reasons, group numbers are issued in the following order:
ODD numbers from 01 through 09
EVEN numbers from 10 through 98
EVEN numbers from 02 through 08
ODD numbers from 11 through 99
As an example, group number 98 will be issued before 11.
The last four digits are serial numbers. They represent a straight numerical sequence of digits from 0001-9999 within the group.