Social Security numbers (SSN) were created in 1936 to track the earnings of U.S. workers. This information was used to determine Social Security benefits.

Today, nearly every legal resident of the U.S. has a SSN. The nine-digit number is used as one of the main ways to identify and gather information about a person.

Before June 2011, the SSN was comprised of three parts. The first three digits were the area number, which were assigned by geographic region. Usually, people on the East Coast had the lowest area numbers and those on the West Coast had the highest.

The next two digits were the group number. The group numbers ranged from 01 to 99 and were not assigned consecutively.

The final four digits were the serial number. This number represented a series of numbers from 0001 to 9999 within each group.

In June 2011, the Social Security Administration (SSA) changed the way Social Security numbers were created. The change is referred to as “randomization.”

It’s a randomized assignment methodology that experts say helps protect the integrity and also extend the longevity of the SSN nationwide. The SSA says randomization makes it more difficult for identity thieves to reconstruct someone’s number by using public information.

There are approximately 420 million numbers available for assignment.

According to the SSA, the nine-digit SSN will eventually run out. The change to randomization in 2011 helps extend the number of SSNs available for assignment for many years.