trouble connecting to a listening service

:mad:Ubuntu 16.04 is running on a TX2.
- running Ubuntu as a server with (nc -l 50000) works fine, I can connect to from a client (nc ip 50000) and exchange messages
- however, when running an application on the Ubuntu server, I am NOT able to connect it from the client (using nc ip 50000), wit Connection Timed Out.
- I have read in another forum (https://support.rackspace.com/how-to...s-with-netcat/), that it might be the firewall, here is the output of iptables on the Ubuntu server:
Chain INPUT (policy ACCEPT)
target prot opt source destination


Chain FORWARD (policy ACCEPT)
target prot opt source destination


Chain OUTPUT (policy ACCEPT)
target prot opt source destination

- Debugging the application shows that "accept" always return NULL even if the client is trying to connect.
Please suggestions?!