DNN does nothing to configure IIS. In a nutshell, your domain name uses DNS to point the specific host, such as "www", to a specific IP address that IIS will respond to on your system. IIS only responds to an IP address and port combination, not a name (though host headers can change this, for DNN you don't need them). When IIS receives a request to a host name that resolves to the IP address IIS is listening on, it will serve the requested page, as in Default.aspx if you request http://www.sample.com/Default.aspx, or to the default document if no specific file/page is in the URL, as in http://www.sample.com/.
DNN takes over when the Default.aspx page is served, processing the requested header (the URL...) and redirecting to the appropriate DNN portal based on the portal alias. If the requested header doesn't appear in the portal alias table, the default DNN portal is served. For both parent and child portals DNN follows this process, which is why having the correct portal alias, as requested, is so important.
In fact, if you have www.sampe.com as a portal alias and someone requests http://sample.com/, DNN won't recognize it. If someone requests your IP address and it's not a portal alias, DNN doesn't recognize it. Most DNN installations should probably have three or four portal aliases, the host/domain, the domain alone, the IP addres and Localhost if the site will be accessed from the server.
Jeff