Understanding NAT address types

I have to agree. By far the best explanation to date.

Thanks Stretch

I've always felt that this whole concept of 'inside' and 'outside' was absurd. Just NAT on the involved interface as traffic enters or exits...sheesh. :)

Anyhow now that NVI is available we can at least drop those useless words from the NAT config, and therefore the concept. The whole inside/outside thing made me want to gag myself with an NM.

Click! And another Cisco concept makes sense thanks to Packet Life!

Not a difficult concept, just Cisco's explanation! Thank you Jeremy!

@mspn

Not that I've ever seen the inside end does not know about the outside.

very good explanation

"location and perspective."

this approach makes it easy to logically think out what the "show ip nat translations" is telling us.

thanks,

This has always been a confusing issue for students and its partly Cisco's fault for making these bad early definitions and sticking to them. It would have been more advantageous to use a 'Actual' SRC/DEST and 'Translated' SRC/DEST type of table.

Oh well.

Jeremy,

ip nat inside source static 192.168.0.10 192.0.2.10

Where does the 192.0.2.10 come from? In your figure I don't see any interface with that ip address??

Cheers,

Robert

Just a thought..

Router B's public address has to be on that subnet that you are SNAT-ing to. In order for this to work it should be like this:

interface FastEthernet0/0
 ip address 192.0.2.10 255.255.255.0
 ip nat outside

Or is there some cisco NAT black magic I am not understanding? SNAT creates a 1 to 1 mapping but the "Inside Global" or "Outside NAT Address" as the rest of the world calls it still has to be routable on that interface. I can't just SNAT something I don't have and expect it to be routable. Or maybe I am just stupid.....

Frank,

Thanks for the comment, it forced me to model the whole thing in GNS3 and really get a good understanding. R2's Fa0/0 does not need to change. The entire model works with a default route from r3 to r2 192.168.0.1 and a static 'ip route 192.0.2.0 255.255.255.0 10.0.0.1' in r1. Add a firewall and this would be a valid B2B environment. i.e a business partner that was using the 192.168.0.0/24 space but not the 192.0.2.0/24 space.

Excellent work Stretch!!!

Steve

The trick question is (for 100 points):

can you use NAT to acces a host that does not have a gateway set ?? :D

ex :

HostB ip 10.10.10.2 /24 ( NO gateway set )
R1 fa0 ip 10.10.10.1 /24
R1 fa1 ip 10.20.20.1 /24
HostB ip 10.20.20.2 /24 gateway to 10.20.20.1

How can you use NAT to ping from B to A.
The ideea is to modify the packets source adress while passing R1. as if the ping was initiated from R1 fa0's interface

Ofcourse a ping from B to anything else except local lan would not work cause no route :D

I went and read the Cisco site on this and it just left me confused, this article was crystal clear when I realised the differences were regarding perspective.

In addition to this the article was only as long as it needed to be where as Cisco's is about 4 times longer.

I understand the stages of address states from original to translated, except for how the (outside local) address is processed.

My theory is that the (outside local) address is seen on the private network to which it is not a member, as the same ip as the (outside global) address.

For the device with an outside global (internet) address, that is connected to its own local network, it would have an outside local address that is different from its outside global address, in its private network.

I envision two private networks facing each other through the internet, and the addresses of each.

But are there internet-only devices with no local network connection, and what of the outside local address then?

Best description for the four terms most often confused.

Inside global: The address of the inside host as seen from the outside Inside local: The address of the inside host as seen from the inside Outside local: The address of the outside host as seen from the inside Outside global: The address of the outside host as seen from the outside