DNS host header mapping

I am trying to host multiple unique domains from a single static ip using the host header function of IIS 5. I can use multiple ip’s with no problem, but am a little confused as to how to denote the host header for www.1.com being different from www.2.com in my win2kAS DNS server. I have gone to IIS5 advanced and setup the host headers as www.1.com and www.2.com respectivly, is the resolution problem with DNS or IIS – where does the header resolution take place?