application behaves errotically if IP address used in URL

[Ramesh]

Hi,
I have an asp.net application in which I have used sqlconnection object,The
application working fine in my development machine, but when I deploy the
build application to our test server,and I use IP address as url it
sometimes throws object instance reference error, the same is working fine
if I use site name. I am not using any firefall to restrict IP lookup,
The authentication mode in my we.config is "windows"
Is any other setting is required?
Thanks
Ramesh