Normally, network address translation (NAT) blocks incoming requests initiated from outside the local network. Inbound traffic is only possible for packets relating to an outbound request (strictly speaking, “related” and “established” packets to an initial outbound connection). In Registered Server mode, your Jamulus Server initiates an outbound connection when it connects to a Directory. From then on, the Directory sends (established/related) connection “pings” (not ICMP pings) to your Server at regular intervals to keep the relevant port(s) open on your router.