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Below is a scenario. It is not a real life situation, but more a hypothetical so I can get clarification of a specific of networking behavior.

Say we have a single PC directly connected via ethernet to a router on the private side. Public side of the the router is connected the the internet via whatever means. Router has NAT enabled, and a single port forward rule. Port forward rule is configured to send incoming traffic from public side to single PC on private side. (lets say to allow TCP pings) PC on private side does not have a default gateway set. Client on public side sends a TCP ping to public interface of above router.

Will the public client get a response ? Does the NAT modify the source IP in the incoming public packet to it's own IP before sending it to the private PC, so that the internal PC knows where to send the response ? Or does standard NAT implementation not modify the source IP of the packet, and therefore the internal PC must use a default gateway ?

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The situation you describe will not modify the source address from the outside to the inside; it only translates the inside address, in either direction.

With no gateway configured on the host, the host will not reply to the ping since it will know the destination will be off the network, and it cannot resolve the layer-3 address to the layer-2 address and has no gateway from which it can use the layer-2 address to create a layer-2 frame.

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  • Thanks Ron. Do you know if NAT standards define any ability for source address modification to occur in in both directions ?
    – OrdinaryOrange
    Jan 17, 2016 at 3:26
  • There are implementations which can do that. For instance, when two companies with overlapping private addressing merge, you can do this with an outside source NAT translation. What you have described, "standard NAT" is inside source, and the port forwarding is inside destination. The third option, outside source will translate the outside source address. Each of the two formerly separate companies can use outside source on their connecting routers. This is something that should only be used as a temporary solution while one side or the other, or both sides, changes the addressing.
    – Ron Maupin ♦
    Jan 17, 2016 at 3:45

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