Where is the fresh air inlet usually located in a home fireplace?

There's a little opening covered by a metal plate on the floor of my fireplace, but I've been reading that it could be an ash dump. When I opened it the other day, a lot of cold air rushed in (along with even more cobwebs). There's a little metal door at the bottom of the chimney outside which I'm uming is the air inlet access door, but other than the opening in the fireplace floor, I can't see where this door leads to. How do I use the air inlet? The fires I build aren't very long-burning, so I'm thinking I must have an air problem.