>>>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.
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>>I believe you have to enter those sub domains in your DNS server or at the ISP level if you don't have a DNS server.
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>But - how do you tell it which virtual to go to? Do you just direct the DNS entry for "subdomain.domain.com" to the address "123.456.789/subdomain"?


In IIS, you would actually need to create a virtual Web (as opposed to virtual Directory). Point it to the physical directory of your choice, and use a "host header name" for this new virtual Web. To do this, once you've created the virtual Web, go to its properties and then:
1) in the WebSite tab click on Advanced...
2) click on Add... under "Multiple identities for this website"
3) type the name ("subdomain.domain.com") in the "Host Header Name" field. You don't have to worry about the other fields in that window.

Then, with your ISP (or your own DNS server if you have it), register "subdomain.domain.com" so that it points to the same IP address as "domain.com" (which is the single IP address for the machine where you performed the above steps).

Now, when a user requests http://subdomain.domain.com, the InterNIC DNS server won't find an entry for it, so it will redirect the request to the DNS server with authority for that domain (domain.com), which it will have record of. This DNS server would be either your ISP's or your own, if you host your own site. This DNS server is the one that will have the mapping described above. Once the request finally makes it to your machine's IP address, IIS handles the name re-direction as needed.

Hope this helps!