Depends on your router, very few can distinguish between subdomains in inbound traffic as all inbound requests look the same to it. Meaning you can only have one site per ip without extra software.
Originally Posted by HodgeHi
If you have multiple external ip addresses and the subdomain can be pointed to one of those that is not in use for a web server the router can use this to distinguish between the traffic.
With only a single external IP using subdomains to direct the traffic would require you to setup ISA (if you have it) to redirect traffic or use an apache server that could reverse proxy to the different sites that are running under different domain/subdomains. You would need to do this anyway it you only have a single external ip and your router is not layer 4 capable.