Tool for converting a single /23 or larger network into a number of smaller networks

Does anyone know of a tool that can take (for example) a single /20 IPv4 network and return all of the /24 or smaller networks it contains?

This just came up at work--at least one of our spam filters; whitelist does not accept networks larger than /24, while the IP whitelists we're dealing with has multiple larger networks in its list--and while it sounds like it could be an interesting project, I want to make sure I'm not duplicating effort if that already exists. A cursory Google search didn't turn up anything, but if something like that exists already I assumed someone here would be able to point me in the right direction.