Wordpress MU defaulting to registration page when using http://domain

After doing numerous multisite setups, I now have a situation that when typing http://mydomain.com (without www) into my browser, my multisite is now going straight to the registration page with the url extension showing as:

http://www.mydomain.com/wp-signup.php?new=mydomain.com

This is also the first time I am using a dedicated IP, so I am not sure if this is what could be causing this behaviour.

Please can someone guide me on what setting needs to be adjusted in the system, so that multisite recognizes my domain without the www

Many thanks