Port Forwarding on ADSL-Router 877 without NAT ?

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Jun 4th, 2009
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I've been searching on how to setup port forwarding on a 877, but the information I find all use NAT for this, like the example below for forwarding to a mail-server:

ip nat inside source static tcp 25 interface dialer 1 25

As the word 'nat' is used, I guess the data on the LAN-side would use the routers IP-Address (eg ; but I need to know the original address (the client's address somewhere on the internet), but how to do such a setup is the information I can't find in the topics mentioning 'port forwarding'.

Can anyone confirm which address the internal server would see ? And if it is the router's address, would it be possible with another config to get the external address ?




Remark: It is a little unfair to post this, as I didn't test if I interprete the config-line correct, but the problem is that I can't bring the current router down if I'm not sure the new one will act equally. So it's more a question during the planning phase...which I want to implement as soon as possible ;)

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Jon Marshall Thu, 06/04/2009 - 10:56
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The internal server would see the real IP address of the client from the Internet.

"ip nat inside source static tcp 25 interface dialer 1 25"

means that the internal server of will appear to the Internet as the IP address of the dialer interface on port 25. The nat statement has no effect on the external client addresses from the Internet.


AlainCloet_ Thu, 06/04/2009 - 10:59
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OK thanks! I feel a little more confident before I start my 'real life trial' :)

Alain Cloet Thu, 06/18/2009 - 11:46
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This works now ... (I didn't find time before to do the complete setup) ... but now I have another problem.

For one of the protocols I'd prefer NAT i.s.o. PAT ; because it should be forwarded to another router, and that one wouldn't know how to reply (the real source can be variable, and the 877 isn't the default gateway for that other router)

Is it possible to combine this (PAT and NAT from outside to inside)? I worked with the Cisco PIX before, and I think that one was easier in setup (NAT & routing were more obvious)...


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