Accessing services from the external network

Question

Does the ICMP request from the external host reach the server inside the private LAN ? if so , how the NAT router knows to which it has to forward the packet in the case of Port Address Translation (PAT) ,when no entry is there in the translation table initially

Answer 1

Typically in the case of ICMP requests, these are answered by the router.

PAT build up a list of forwards based on the destination IP and port, but also the source port. When the response is received it can identify where it goes based on the same characteristics it's already recorded from the preceding related outbound packet. If there is no known forwarding for the packet, then the router itself answers (or discards) it.