Saturday, May 19, 2012

Configuring NAT - Static 1-1 NAT

In this series of NAT blogtorials, we will start with a simple static 1 to 1 NAT mapping. Consider the simple topology below and let's dive into the world of NAT.
Follow the screenshots :)

Get the interfaces configured on all the routers. IP, description etc. 

Now let's define an inside interface and an outside interface on the NAT boundary router (R2). Inside interface are usually private IPs, however not always.

We need to now configure R2 to NAT anything coming in (source) with IP address of (R4 fast1/0 IP) to (R2 fast1/1 IP). There by creating a 1 to 1 NAT. is NAT'd to

When I first learned NAT these 4 terms always confused me. So I am going to try and water it down :)
  • Inside global - This is the address as the outside sees it. 
  • Inside local - This is the address that other inside hosts can see. 
  • Outside local - This is the address that the inside is trying to go to.
  • Outside global - This is how the outside sees it. 
An example should clear things up. Let's ping (R3 fast1/0 IP) from R4 (

Notice that the packets sourcing from R4 are NAT'd to and the pings succeed.

Some NAT troubleshooting commands are:

Many more articles to come so stay tuned. "Join this site" on the right / click +1 below ------->

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