sub domain

Hello all.

I'm trying to make it possible for someone to visit my web site if they
enter http://mydomain.com instead of http://www.mydomain.com. I talked to my
host (Windows2003) and they already set it up in their DNS and they told me
that I should configure it in my IIS by clicking the advanced button next to
the IP address selection. I'm supposed to add the same IP address and under
host header value I'm supposed to add mydomain.com.

I tried this and it still doesn't work. My host support did not sound
confident when he gave me these instructions so I'm hoping that someone here
could either verify the instructions to be correct or steer me to the
correct path to get this done.

Thank you,

- J