I think your concept of a hop is a bit off. Each hop represents a stop at a computer, not and interface. So the first hop is his router, when it generates the ICMP reply it should send it out on the interface closes the computer it's sending it to. Therefore the address on the packet should be that of the internal interface. Traceroute then ups the TTL and the router sends it on the next host that does the same, that packet has the address of the interface on that system that is closest to the original sender.

NAT is the rewriting of address and port fields for packets, they still have to be routed before/after they are rewritten. It shouldn't have an effect in this case.

In reply to Re^4: How to identify router IP address by Ven'Tatsu
in thread How to identify router IP address by dba

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