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NAT Questions

I'm studying for the CCNP routing exam and want to get a firm understanding of NAT. I have a couple questions that a few of you might be able to answer better.

  1. What does the router look at first the routing table or the NAT table? And does it matter which way the traffic is coming into the router?

  1. I undertand what the Inside local/Global address and how they are used, but I'm uncertain of the Outside local/global address and why you would use them. Is the outside Global address a public (routable) address of a remote host? I.E Webserver in a remote network?? Not sure what the Outside local address. Can somone provide examples.

  2. When setting up Static Port Mapping:

router (config)# ip nat inside source static tcp 10.0.0.1 80

200.152.14.56 80

I know this will setup any address trying to reach 200.152.14.56 on port 80 to be sent to 10.0.0.1. This is incoming, does anything happen when this local machine goes out to the internet? I'm not sure I tottaly understand this area.

  1. When setting up the intferfaces for NAT by applying the "inside" and "outside" to the interface, what does this essentially do?

Any help will be much appreciated, also any study material that you can guide me on this subject will help too.

Thanks

Matt

Reply to
Matt
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You can find out the answers in the following papers:

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KPLAB

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- Free CCNA and CCNP Study Guides

Reply to
kplab

Depends where NAT is configured. If it on the inbound interface, then it will NAT before the routing table. If its outbound, then it will route, then NAT.

You need these for overlapping networks, when merging two private networks that currently occupy the same space, and there are servers on each that much be accessible by clients on the other. Don't confuse "Global" with "Publicly routeable". This is only in the (common) case that "outside" really is "outside".

Try it. I think you'll find that a static mapping implies both ways. The above may not actually work as return traffic may not be NATted.

Defines where each interface. Kinda fundamental to NAT. Go back to the text book, draw some pictures.

Reply to
yamahasw40

I feel like I'm just about to understand this topic but can't completely wrap my head around it. What is the difference between GLOBAL/LOCAL and INSIDE/OUTSIDE??

What would be the difference between the below statements:

ip nat inside source static 10.0.0.1 170.23.5.10 vs ip nat outside source statice 200.45.121.45 10.1.1.5

I think I understand that the inside will translate the 10.0.0.1 address to the 170.23.5.10, but what is the purpose of the next line. I'm getting stuck on the outside part and I think I'm confusing the local/global with inside/outside. Usually I grasp things pretty quick but for some reason I'm having a problem with this.

Thanks in advance,

Matt

Reply to
Matt

[snip]

this is one of the clearest explanations from Cisco

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ip nat inside source blah: translates the source of IP packets that are traveling inside to outside

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

ip nat outside source blah: translates the source of the IP packets that are traveling outside to inside

*AND* translates the destination of the IP packets that are traveling inside to outside
Reply to
Hansang Bae

How does this look:

IP NAT INSIDE SOURCE

(SOURCE) 10.0.0.1 ------> 69.18.152.5 inside-------->Outside

10.0.0.1
Reply to
Matt

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