Uh, the answer is right in front of you. ... In the back, typically.

Here's an idea, whip out the wrench and see if the front and the back are different. If they're different, as Mr. Hairnet suggests, you might be able to get the rear nut to thread onto the front, but you're not going to get the nut from the front on the rear axle. Go from there.

If you have a socket set, see which fits better- 15mm or 9/16"