Since IRLP behaves like both a client and a server (from a network perspective), it requires some port forwarding if it's installed behind a NAT router.  Since most ISPs provide only 1 public IP, it is very common to have a router configured to provide NAT (network address translation) services for the local network so that multiple devices, PCs, etc. may access the Internet.  If the IRLP Node computer is on a local network behind a router as explained, then the router will need to have some port forwarding configured for the internal IP to which the node is assigned.  IRLP requires the following:

UDP Ports 2074 - 2092 (inbound from the Internet - carries the audio payload for channels 0 to 9)

TCP Port 15425 (inbound from the Internet - control connections)

There are hundreds of different types of NAT routers so other than to suggest The Port Forwarding Site, I won't attempt to cover router configuration specifics.