getting the normal vector from each side of a box collider

I want to get the normal vector from each side of a box collider.

I found that if I can create a plane for each side, i can find its normal vector.

Problem is Im having some trouble figuring out how to construct a plane given the extents of the bounds of the collider.

Additionally I need the normal of the plane constructed to be pointed into the collider, not out of it.

Any insight on this?

Thanks,