The result is that computers behind a NAT can no longer automatically receive connections. This is because other computers from the outside only know the IP of the NAT, and the NAT does not know to which computer an incoming connection belongs.
If you are in such a situation you have to configure your NAT (usually your DSL router) to be able to receive incoming connections.
If you don't do this you will only be able to connect to users that are able to receive incoming connections, in other words you are passive.
To be able to take incoming connections despite being behind a NAT, you need to reserve a
port on the NAT for your PC. This means you tell your NAT if a connection is coming in on
port X then its for PC A.
This is usually called "Port forwarding". To get instructions
on how this works with your NAT, simply use a search engine and search for the brand of your
router/NAT in combination with "port forwarding".
In Jucy -> Preferences -> Connection Settings you then have to enter the forwarded port under TCP and UDP. For encrypted connections you need to forward another port to your PC and enter this port under TCP+TLS.
Note that most router do this forwarding by transport protocol (UDP/TCP). This means you might have to create an additional forwarding rule for the UDP port even though you already set the NAT to forward the same port number for TCP.