I would go ahead and cast on the 56 stitches (if you're making them top down n not toe up) then I would decrease evenly around to the 48 stitches for the foot and ankle parts.
I would decrease right under where the calf muscle is. Or for a more gradual decrease I would decrease half my stitches right under the calf muscle work a few more rounds (say 5 or 6) then decrease the other half to get to the 48 stitches. Then work your sock as normal.
If you're working a pattern into the sock, that'll take a little bit more math but it can be done.
I hope that helps hun!~!!