To force a specific domain to be used when your site can be accessed by several, a tidy way is to use mod_rewrite. This lends itself very well to password protected directories that have images referenced by a specific domain and you don’t want multiple password prompts.
RewriteEngine On RewriteCond %{HTTP_HOST} !^example.com [NC] RewriteRule ^.*$ http://example.com%{REQUEST_URI} [R,L]