You need to setup your domain name as *.YourDomain.com.
The * will tell the routers to send anything.YourDomain.com to your domain.
You will know this has been done correctly if you can ping your subdomain.
After that, you just setup IIS to filter and send to the appropriate site.

Chris



>What do I need to do to a virtual directory to make the web address "subdomain.domain.com" get directed to it? I thought it would automatically do this, but it's just defaulting to my default web site instead.
>
>TIA