configure a sharepoint 2013 site for external and internal access

I need to configure a local install of sharepoint 2013 so that users can access it internally and externally using windows/AD authentication. The internal and external addresses are different.

I have bound an external ip to the domain for external access.

I have created Alternate Access mapping, and bound the host header but I get a file not found message for external access.

Have I missed something here? why the error and how can it be fixed. Step by step process would be appreciated.