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New Member

what happens if.....?

just a senario i came up with in my head that i was curious about.

router1

s0=ISP internal network

s1=160.160.160.1/24

router2

s0=160.160.160.2/24 (ip outside)

eth0=192.168.100.0/24 (ip inside)

router1 is owned by an ISP. and router2 is owned by xyz.corp.

the ISP gave xyz.corp a public IP of 200.200.200.0/24 and router2 will only allow an IP from 200.200.200.0/24 to the internet through its internal network.

before the 200.200.200.0/24 network used to be on router2's eth0 interface and all clients off of that interface had public ips. now xyz.corp wanted to NAT using a private IP range of 192.168.100.0/24.

the router is overloading s0 to 200.200.200.1/24. router1 has a route that says 200.200.200.0/24 is out its s1 interface, the way it used to be setup that was true. technically it is true now, but 200.200.200.1 isn't bound to an interface its just an ip that 192.168.100.x is NAT/PAT to. what happens? the 200.200.200.1 address is really on the subnet that the 160.160.160.0/24 subnet is on. clients will send out a request to cnn.com 192.168.100.30 will be NAT/PAT to 200.200.200.1 and then off to the ISP router, the ISP router will see that 200.200.200.1 is allowed to the internet and will allow it through, cnn.com will respond to 200.200.200.1 and the response will go back to router2, router2 will see from its table that 200.200.200.x is down s1 and will forward it, router1 will get it and do what? what happens first when the packet comes into the interface?

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080133ddd.shtml

lists the order of operations for NAT. routing happens first, the default route on router2 says route everything out s0. router2 doesn't have a route to the 200 network because it really doesn't exist and will route it back to router1, router1 will route it back to router2.....till TTL exprires.

would have you have put a NULL interface in, assign it an ip and point a route to that one router2?

anyways just curious what would happen. i was talking with my friends and somehow we ended up with this senario....and wondered what would happen if...

thanks.

2 REPLIES
Bronze

Re: what happens if.....?

I'd imagine that NAT table is checked before the routing table for return traffic. But someone can correct me if this isn't the case.

New Member

Re: what happens if.....?

thanks.

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