AS Chemistry, using moles in calculations :(?

We're doing calculations in chemistry at the moment, and its complicated but i think i understand most of it, there's just one thing that i don't understand. When you're working out the concentration of something from the moles, you do the 1000/vol times the moles. I would have thought you'd do the volume/1000 to get it into dm^3? Also you do the vol in cm^3/1000 when finding the moles from concentration which makes sense because that's how you turn the cm into dm but i don't understand why its 1000/volume to find the concentration from moles