An old chestnut goes like this:

On a 3x3 chessboard, there are knights in the four corners.

The knights on the left are white; the ones on the right are black.

 3  N.n
 2  ...
 1  N.n


The knights move like normal chess knights. How many moves does it take to swap the black and white knights?

If consecutive moves by the same knight count as one move, what is the smallest number of moves possible?