Help Request - Double NAT, "ip nat inside" and "ip nat outside"

flying.ghost

Hi friends,

I am new to techexams and currently preparing for CCNA exam. I am using Cisco documentation and some other technical websites for learning NAT on Cisco Routers (on GNS3, 7200 VXR IOS image).

Configuring Network Address Translation: Getting Started - Cisco

Inside Local, Inside Global, Outside Local, Outside Global

How to configure PAT (Port Address Translation or NAT overload) in a Cisco Router

What I understood from these documentations are if I am using NAT inside my company LAN, two times NAT happens. One at my LAN edge router and other at the other network edge Router. This is clear.

But I saw in some other documentations about Double NAT. Below are the links.

Using NAT in Overlapping Networks - Cisco

https://supportforums.cisco.com/discussion/11337636/difference-between-double-nat-and-twice-nat

What is Double NAT? How this works? What is the difference between NAT and Double NAT?

Another doubt is, what is the difference between the commands "ip nat inside" and "ip nat outside" ? I had used only "ip nat inside".

Please help to clear this and please explain the concept of Double NAT.

Thanks in advance.

ImYourOnlyDJ

I'm pretty new and studying for the CCENT myself, but I believe double NAT is when you have NAT inside of a NAT. For example hooking up your cisco router to your home router, then having a seperate network (for example your lab) that the cisco router would route. Hopefully somebody else can confirm this

http://slinkware.com/help/double-nat/ After some googling I think this might help

theodoxa

ip nat outside would translate an outside address to an inside address. I believe this would be used for connecting networks with overlapping IP Address ranges. I've never used outside NAT, but it is covered in the CCNA books, so I would learn it.

As for ^^^, one cool use of NAT is being able to route to an otherwise inaccessible network. For example, most home routers either don't support any routing other than a single default route (WAN) and a single connected route (LAN) or only support routing on the WAN side (D-Link), meaning that there is normally no way to route traffic to/from another network placed behind said router. What you can do, and I used to do in my home lab, is NAT at the edge of your lab network to addresses on your home network. This way, all the routers, servers, etc...appear to be on the network directly attached to the LAN interface of your home router. Your home router would forward any packets to those addresses to your Lab Edge router who would translate the home network addresses to lab network addresses. You can do a simple NAT overload of the outside interface of the Lab Edge router if all you want is for the lab to be able to get out to the internet.