The Social Security number's first three digits -- called the "area number" -- is issued according to the Zip code of the mailing address provided in the application form. The fourth and fifth digits -- known as the "group number" -- transition slowly, and often remain constant over several years for a given region. The last four digits are assigned sequentially.
Until 1988, people typically had six months after the birth of their child to file for a SSN. Starting in 1988, the Social Security Administration (SSA) urged people to file for a SSN shortly after birth.
Additionally, there is a publicly available "Death Master File"
As it happens, the researchers said, if you're trying to discover a living person's SSN, the best place to start is with a list of dead people -- particularly deceased people who were born around the time and place of your subject. The so-called "Death Master File," is a publicly available file which lists SSNs, names, dates of birth and death, and the states of all individuals who have applied for a number and whose deaths have been reported to the Social Security Administration.
If someone knows when and where you were born, particularly if you were born after 1988, they can easily guess the first five digits of your SSN. In the age of social media, this information can be easy to come by.
For anyone born before June 25, 2011, it is important to keep in mind that the last four digits of your SSN are the ones that are hardest to guess. Guard those last four digits in the same manner that you would guard your full SSN.