How to determine what ports an application uses

I have a network that uses a server in my basement at a gateway. It runs iptables. All net traffic in the house to the outside passes through this. Is there a way to monitor the traffic to see what ports an application needs? From time to time we run apps that need particular ports open ... e.g., our local library doesn't use port 80 ... I'd like to be able to see what ports an application is using. I've tried netstat but cannot see what ports are being used. Any suggestion on how to do this? (Right now I google until I find what ports an application uses ... but as more devices appear in the house this gets to be a pain.

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