How can I update my domain ?

[DeeBoo]

Hello Guys

I am a new user in ubuntu 8.04 server and I have a small problem

I have finished setup ubuntu 8.04 server and I am left with a small problem:

I installed webmin in my server and in some sites in my server it is giving me my network IP instead of my domain , for example :

In one of my sites it is giving me this address :

http://192.168.2.3/up and it should be :
http://www.mydomain.com/up

so how can I solve this problem ?? this is the only problem I have