Setup AD with Internal and External Names

I have always know you should use .local for you internal and .com for your external. When creating the AD I would assume you would use the .local, but after that where do you set your .com name.

I have a small network using DHCP, DNS, IIS.
I host all of our email, website. I am rebuilding my entire setup, so I want to set it up correctly with two different names.

Any direction would be seriously appreciated.