From Kathleen Pender in today’s San Francisco Chronicle:
…How does the government assign these numbers? Is there anything you can tell about a person from his or her number? And in using the last two digits, is the IRS favoring one group over another?
It turns out that the first three digits – called the area number – are based on the ZIP code of the mailing address listed on the application for a Social Security number. It can usually tell you something about a person’s origins.
Within each area, the next two digits – called the group number – are assigned in consecutive order as applications come in. Instead of going from 01 to 99, however, the Social Security Administration follows an odd-even numbering pattern. Within each group, the last four digits are assigned in consecutive order from 1 through 9999.
See more here