Short but useful tip:
If you want to test if a port is open on Linux (e.g. all firewalls were opened) but you don’t have the listening application installed yet, you can use nc/netcat.
On the server side do:
# For TCP nc -l -p $PORT_NUMBER # For UDP nc -l -u -p $PORT_NUMBER
On the client side you can now connect to the listening port:
# For TCP nc host.tld $PORT_NUMBER # For UDP nc -u host.tld $PORT_NUMBER
This is basically a telnet chat, so you can send strings which should then be received on the other side.