Application fails to receive tcp message on port

How can a range of ports be forwarded?

I am programming a Windows application that needs to "listen" to messages from its "sister" application running on another computer.

The messages are tcp messages, and they come in on any one of various ports.

(I don't think I can specify which port, since the .NET Frameworks assigns the port at runtime within a range of approx. 48,000 - 50,000.)

The application does succeed in "sending" traffic out to a port on the other machine, but the application is not receiving traffic from the assigned port.

I don't think it's a firewall issue, since it looks to me as if the firewalls on the Mac and on Parallels are turned off.

Port-forwarding seems to work if the port is "known", but how can a range of ports be forwarded?