John Higdon wrote:

It is rather difficult to "make up" and SSN. Like credit card numbers,
they have checksums. A bogus SSN will drop a flag the moment it enters
an SSN-savy computer.

It's easy. Anybody who knows anybody shady (or geeky) knows how to do it, no
checksums involved, just a few simple rules. In fact, Wikipedia has an entry:
http://en.wikipedia.org/wiki/Social_Security_number

But back before all this hoo-hah, people made up fake SSNs by simply
rearranging the last 4 numbers of a legit SSN.