NAT : Inside and Outside source

May 20, 2005 5 Replies

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Bernard Herickx 21 years ago

You know the "ip nat inside source list acl_id pool pool_name" command. It says at least that the range of inside (private or local) addresses defined by the acl list will be translated into outside (public or global) adresses defined by the NAT pool. But what special does says "inside source"? After all, the command could be shortened into "ip nat list acl_id pool pool_name" to establish the association between inside (local) and outside (global) adresses.

However "inside source" must have a special meaning because beside the "ip nat inside source" there is an "ip nat outside source" command. So what does both inside and outside source mean? Does the word "source" refer to the source of a transport (TCP) session ?

As a secondary question, what purpose is the "ip nat outside source" command for?

Thanks for any clarification on this subject.

Bernard.

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Doan 21 years ago

I think source here refers to the source address (SA) of an ip packet.

It's for packets that originate from the outside. It

translates the source of the IP packets that are traveling outside to inside

translates the destination of the IP packets that are traveling inside to outside

And for ip nat inside source list, it's for packets that originated from the inside. It

translates the source of IP packets that are traveling inside to outside

translates the destination of the IP packets that are traveling outside to inside

It's important to note that they both do basically the same thing but the order of operations (like routing) is different depending on if you are natting from inside-to-outside vs outside-to-inside. Here is a good link on this issue:

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Doan

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Phil Watkins 21 years ago

inside & outside are just labels, it doesn't matter which way around you put them. hope that's enough to help

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Bernard Herickx 21 years ago

Thanks Doan.

Bernard.

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Bernard Herickx 21 years ago

Finally reading

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to undertand that

the "ip nat inside source" purpose is to make an inside address appear as a genuine outside address when the packet travels and is seen on the outside network.

the "ip nat outside source" purpose is to make an outside address appear as a genuine inside address when the packet travels and is seen on the inside network

Does it make sense ? ________________________

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Hansang Bae 21 years ago

Yes, that is correct. "ip nat inside source" will translate the SOURCE address as it travels inside (clean side) to outside (dirty side). And it translates the destination address as it travels outside to dirty side.

See

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for a very clear example of what's going on.

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