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Desperately trying to understand Nat

Posted on 2004-08-10
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Last Modified: 2010-04-17
I'm setting up a Cisco 2600 series router that will connect two sites together with a private line.  Each client wants to use their own block of IPs to access certain systems, so client #1 has dedicated 12.4.xxx.xxx (not sure why but the customer has chosen to use public IPs)to access customer #2 fileserver and 12.4.xxx.xxx to access customer #1 mail server.  Customer #2 wants to translate their IPs to a private IP address before allowing it to route internally.  So the picture looks something like this:

Customer #1 ---->12.4.xxx.xxx  ---->Customer #2 ---->192.168.xxx.xxx ---->System #1

Call me simple-minded but natting has always been a little confusing to me.  I'm not sure where to do the natting here, on both routers or just one?  Am I changing the source Address or the destination address?  I hope this makes sense.....
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Question by:sunny10
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Quetzal earned 125 total points
ID: 11771374
NAT will only apply to Customer #2.    You can configure the router in two ways.  (1) associate a unique public ip addr with a private addr, (2) associate a single public ip addr with multiple private addr's but forward specific ports to specific private addr's.  You could mix both modes too.  Method #2 is commonly referred to generically as NAT.

In method 1, the private ip addr from Customer #2 are changed to the public ip addr.  Traffic to Customer #2 will change the destination addr.  Traffic from Customer #2 will change the source addr.  The source and destination ports will not be translated.

In method 2, the combination of the private ip addr/port number from Customer #2 is changed to a unique combination of public ip addr/port number.  Traffic to Customer #2 will change the destination public ip/port to private ip/port.  Traffic from Customer #2 will change the source addr.  Port forwarding affects only traffic to Customer #2.  Traffic to the destination public ip/fwded port will be changed to the private ip/port specified in the forwarding rule.  Note that the source and destination ports are translated.
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by:Quetzal
ID: 11771394
In method 2, the third sentence should read: Traffic from Customer #2 will change the source private ip addr/port to public ip/port (where the public ip port will be unique to all open connections at the time of the packet transmission).
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by:sunny10
ID: 11849140
ok, so I created a route-map on customer #2's router.  the route map looks at the source and destination address coming from customer #1 and hands out an address based on the access-list.  I placed 'ip nat inside' on the E0 interface and 'ip nat outside' on the serial port.  Do I need to create a route-map on customer #1's router as well, or does the ip source outside take care of translating it back to its original source address?  The route map isn't working right now but I think it has to do with the firewall and not the list (I think).
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