xclock fails with "Can't open display:" if DISPLA

[muki46]

I use knoppix 5.1. (LInux 2.6.19). When DISPLAY is set as :0.0, xclock works fine.
When DISPLAY is set with MyIpAddress:0.0, I get Can't open display: error message,
and xclock fails. I am a normal user (no super user).
The same thing works very well on mandriva linux. I could not find any useful hint ind google.
Thanks in advance for any useful hint.