[TYPO3] IP address in URL

[Gunda]

Hi everybody,
We have a peculiar problem and have been unable to find any help in many hours or 
searching through forums and documentation.  We are unable to get Typo3 change our host 
server's IP address (i.e. 192.168.1.1) to the web domain name (mydomain.com). We can 
type www.mydomain.com into the browser address, the site is loaded but it shows the IP 
address instead of the domain name in the URL. Our ISP says we need to change a Typo3 
setting. Which file needs to be edited or what settings need to be altered? 
Thanks for any help in advance!
Gunda