windows authentication doesnt always work

This is for an intranet site.

If i type the ip address, it makes me enter a password, but if i type in the machine name, it lets me right through.

http://192.168.XXX.YYY -> password protected
http://myServerName -> no password

does anyone know why this might be?ok, i kinda have it figured out now.

If i type the ip address, it does checks the password.
If I use the serverName, it actually takes the login that i used to log onto the computer.

I'm not sure if that's what it's supposed to do, but it actually works out great for me.