What I need to do is open a new port for each user (connection). How, programaticly, do I check if a certain port is open?

You could try telneting to port 139 from an outside machine. If you get anything other than an access denied, you likely have an open port ...

How to Check if an Ethernet Port Is Configured for a Dynamic IP. Every device connected to a network must have its own Internet Protocol address to communicate with other devices.

What is The specified port is already open VPN error? While we use a VPN on our computer to connect to the internet securely, it uses a TCP port 1723 by default to connect to the network.