I apologise in advance if the question doesn't make sense, please let me know.

I've got a small LAN (~10 Virtual Servers) using Win Server 2008 as a DNS server. This is behind a smoothwall express 3.0 firewall with ports forwarded for specific services. I have a domain (123-reg) with the NS's that of afraid.org (DynamicDNS) and subdomains pointed to my (Dynamic) IP address e.g.

subdomain1.example.com -> 123.456.789.101.

I think that adequately explains my set up.

My question is, am I able to have subdomains e.g. subdomain1.example.com only point to a specific local host? Like so:

subdomain1.example.com:80 -> firewall(external facing) -> server1.example.com:80
subdomain2.example.com:80 -> firewall(external facing) -> server2.example.com:80

I don't actually necessarily want to use port 80, otherwise I would just use VirtualHosts on apache, it is just an example port.

Currently I can use either subdomain1.example.com OR subdomain2.example.com and they will both point to server1.example.com:80

I do not have to stay using Win Server 2008 for DNS, I am more than happy to move over to BIND if needs be, it was just easier to use Win Server 2008's DNS.

I do not know if this is even possible, I have a feeling it isn't as I've only got one external IP address but any information is useful!