How to direct incoming traffic to subdomain?

I have 3 servers, each with direct connections to the internet (host names
and IP's are just examples):

- mydomain.dyndns.org (100.100.100.100)
- SERVER1.mydomain.dyndns.org (100.100.100.101)
- SERVER2.mydomain.dyndns.org (100.100.100.102)

The primary server -- mydomain.dyndns.org server (running Windows Server
2008) -- is configured as a DNS server. It is registered with the Internet
DNS via dyndns.org. The subdomains are not registered on the internet (not
possible via dyndns, but not sure if it's required).

I want to host a Web sites on server1 and server2. I can see requests to
server1.mydomain.dyndns.org coming in to the primary server, but not being
routed to server1.

There are Host (A) records for server1 and server2 configured in DNS on the
primary server. I thought that was all I needed, but I cannot get incoming
traffic routed to the appropriate server.

Is it possible to have requests routed from the primary server to the
subdomain server? If so, how would I configure this? We're just trying to
get a test environment set up for some development testing, and I'm a bit of
a noob when it comes to DNS, so any help would be appreciated.

Thanks,
David