MVC 4 Web API CORS won't work over Web

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CORS

Hi,
I have a Web API that implementing CORS this way and this is working fine when I call the API on the same network making a CORS Jquery request to the API but this won't work when I try to access it over domain name. So it works when I do

http://12.12.12.13/MyAPI/MyControler/MyAction

but won't work when I do

http://api.mydomain.com/MyAPI/MyControler/MyAction

even if 12.12.12.13 and api.mydomain.com are the same server... Why? I also did check the headers using Fiddler and the only difference between the two request (internal and external) is on the answer headers there is no allow-origin header when I try to call the api using domain name... Why the response header change depending of if I am internal or external?

Thanks for help :)