Can not view domains internally but external works fine.

When ever I try to view a hosted domain internally I get nothing, I have to use the ip/path to get to any sites I want to view, externally though, everything works perfect.

I am using ubuntu / apache2/ php5/ mysql and the like along with webmin.

Anyone have an idea where to start? What other important information did I forget to relay?

P.S. If i try to type in the domain it of course trys to hit the external IP of the server, thus not going anywhere since I am internal.