It probably comes as no surprise that Social Security numbers aren't randomly assigned. Some people wonder just how non-random the number assignments are, claiming that you can figure out quite a bit about a person just from his or her Social Security number. You really have no need to worry: what little information is buried in your Social Security number is very difficult to dig out.
How They Create New Numbers
Social Security numbers come in that familiar three-block form
The first block of three digits is an area number determined by the ZIP code of the mailing address used on the application for a Social Security number. This number will never be "000" or "666." This leaves 998 possibilities. There are over 40,000 ZIP codes, so not every ZIP code will get its own area number.
The second block of numbers is a group number. For administration reasons, the numbers are assigned in this order:
Odd numbers 01 through 09
Even numbers 10 through 98
Even numbers 02 through 08
Odd numbers 11 through 99
Like the area number, this block of numbers will never be all zeroes.
The final four digits are a true serial number; they're assigned sequentially, and again the block will never be all zeroes.
How does all this fit together? Here's an example: The first person to apply for a Social Security number from area 001 will get the number 001-01-0001. The second will get 001-01-0002. The 9,999th person to apply for an SSN in area 001 will get 001-01-9999; the next person will be assigned 001-03-0001.
Many years down the line, the 999,900th person (because that last serial number block will never be all zeroes) to apply for an SSN in that area will be assigned 001-99-9999. That exhausts the numbers for the 001 area. The next unassigned area number will then be assigned to that area, and the first person to apply for an SSN in that area will get XXX-01-0001, and it all starts over again. As of August 2007, the highest assigned area number was 772.
What Do They Do with Old Numbers?
The Social Security Administration does not recycle old SSNs. That makes sense considering that SSNs are used to identify a single specific person both in the past and the present.
According to the Social Security Administration, over 440 million Social Security numbers have been assigned — that's less than half the numbers available — and every year, they assign approximately 5.5 million more. So we have plenty more numbers available for generations to come.