My explanation has been "the resolving of an IP is a matter of your ISP. They should obtain the IP either sending a domain name with or without www. When you directly send the IP you bypass the server IP resolver. My server only understands of IP figures. Therefore this has nothing to do with my server".
I ask, am I right? or there is another possible reason for that missfunction of my client domain? I have to point out that this problem is not stable... sometimes everything works fine, sometimes it does not.