Question

Consider the scenario below in which three hosts, with private IP addresses 10.0.1.13, 10.0.1.15, 10.0.1.20 are in a local network behind a NATted router that sits between these three hosts and the larger Internet. NAT Translation Table WAN side addr LAN side addr 35.122.196.219:517 10.0.1.27:3435 10.0.1.13 2 S: D, 10.0.1.27 135.122.196.219 10.0.1.15 4 3 10.0.1.20 IP datagrams being sent from, or destined to, these three hosts must pass through this NAT router. The router's interface on the LAN side has IP address 10.0.1.27, while the router's address on the Internet side has IP address 135.122.196.219 Suppose that the host with IP address 10.0.1.13 sends an IP datagram destined to host 128.119.163.190. The source port is 3435, and the destination port is 80

Answer 1

I've answered a similar question sometime back. Not sure if it's the same person asking it. Hence answering it again.

In Step 1 when the machine 10.0.1.3 at port 3435 sends a packet to 128.119.163.190, it is actually sent to the router first at it's internal (LAN) IP and port which is 10.0.1.27 and 3435.

Then in Step 2 router sends it to the actual IP 128.119.163.190 and port 80 from it's WAN IP 135.122.196.219 and port 5171.

Same steps are repeated (in the reverse order) when the machine 128.119.163.190 sends a packet back to 10.0.1.3