URL changes when using domain alias

I have installed ZenCart on a server at my hosting provider
http://server2837.hostingcompany.nl
several aliases are set up for my ZenCart shop
when entering the aliasname in the browser it shows as:
http://www.myalias.nl but when I move to a page I see a URL starting with
http://server2837.hostingcompany.nl
This is not what I want, can I change it somewhere in the config ?

Zen Cart builds all its URLs from the information you specify in your /includes/configure.php file.

Thanks Dr Byte, I got confused after the main domain at a webhost expired and at the same time I did an upgrade of ZenCart,
must have changed the configure.php by upgrading. Thanks again.