Problem With Windows Authentication

I have a problem with windows authentication.
I want that domain users could access to my website without specify credentials.
in my web.config i have specified:

<authentication mode="Windows"/>
<authorization>
  <deny users="?"/>
</authorization>

in IIS 5.1 i have checked in directory security only integrated security.
i'm testing web appl. in the same machine where i work: windows xp IIS 5.1
and i'm logged on windows xp as MYDOMAIN\USERNAME.

When i visit website if i specify my site with ip: http://192.168.xx.xx/mySyte
asks me credentials and i could access only if i specify my credentials which are the same i'm logged to windows.

doesn't ask me credentials only if i access with: http://localhost/mysite

why this behaviour? Any ideas

Thank you
Andr ...