How does it know?

Answered Question
Aug 14th, 2007
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i have two routers sitting on the same LAN. the first router provides the interface with ISP and PAT for outgoing traffic - router two has the static NAT entries for our www servers. Since both routers have routes to the www servers (10.10.10.x)how does the first (ISP) router know to send the traffic from the ISP to the dedicated NAT router.

if we have a www server that needs a static NAT we set the default gateway to the static NAT router rather than the ISP facing router.

i cant figure out why the ISP router doesnt send the packet directly to the www server and by passing the NAT router.


thanks in advance.

Correct Answer by Jon Marshall about 9 years 11 months ago

Hi Jerry


Just to confirm - the static NAT entries are done on router two.


So router 1 & router 2 both have an interface into your LAN 10.10.10.x. But when the traffic is coming from outside to your www server it is a natted address. Router 1 receives the packet but the destination is not a 10.10.10.x address because it hasn't been natted back by router 2 yet.


So router 1 arps out for the NAT address and router 2 responds, the packet gets sent to router 2 and then router 2 NATs to back to 10.10.10.x and sends it to the www server.


Hope this makes sense


Jon

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Correct Answer
Jon Marshall Tue, 08/14/2007 - 08:43
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Hi Jerry


Just to confirm - the static NAT entries are done on router two.


So router 1 & router 2 both have an interface into your LAN 10.10.10.x. But when the traffic is coming from outside to your www server it is a natted address. Router 1 receives the packet but the destination is not a 10.10.10.x address because it hasn't been natted back by router 2 yet.


So router 1 arps out for the NAT address and router 2 responds, the packet gets sent to router 2 and then router 2 NATs to back to 10.10.10.x and sends it to the www server.


Hope this makes sense


Jon

jerry.mcrae Tue, 08/14/2007 - 08:58
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"So router 1 arps out for the NAT address"


router 1 arps looking for the private ip of the unNATted public ip?


thanks.

jerry.mcrae Tue, 08/14/2007 - 09:07
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i got it!


(ARP) is the standard method for finding a host's hardware address when only its network layer address is known.


thanks.

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