Log on to your portal as host, and then go to Host | Portals.

Use the pencil image to edit your default portal. Once there, scroll to the bottom and add a Portal Alias aka HTTP alias that looks like www.mydomain.com or localhost/myDNN or whatever is correct in your case.

Then you should be able to log off and navigate to your portal with the new alias. Once you've done that, you can go back to the same place if you want and remove the IP address.

If I understand fully what you are saying, that is.

You can actually have many aliases for the same portal as long as they all point there (domain names and virtual directories)

Even if a domain name/virtual directory does point correctly at DNN, DNN won't know what to do with it unless you have the alias set up. (Note: Different portals cannot share the same alias.) DNN looks at the alias, and then gets the PortalID for the portal that uses that alias, and from that point gets the site content by using the PortalID.