Tuesday, May 22, 2012

Configuring NAT - Dynamic NAT with PAT

Third post in this NAT series is "Dynamic NAT with Port Address Translation". Port Address Translation (PAT) works by translating not just the IP address but the ports associated with the connection. It was mainly created to allow many-to-one NAT, so a single public IP can be shared between 1000s of hosts.

Topology we will be using is similar to the ones from my previous posts on this series. 
Should be a breeze to configure the interfaces with IP and description etc or you can look at my previous NAT posts.  

Relevant configurations regarding the NAT statements are posted below. 

  • Define the NAT 'inside' interface. See previous posts if you need a refresher but basically 'ip nat inside' on R2 fast1/0. 
  • Define the NAT 'outside' interface. See previous NAT posts if you need a refresher but basically 'ip nat outside' on R2 fast1/1.  
  • Create an access-list to match the subnet(s) to be NAT'd.
  • Create the NAT statement.
Show commands to verify: 


Notice that I initiated two telnet sessions from R4, each with a different source port and you can see the associated two different matching entries in the NAT translation table. 

Troubleshooting tips:

  • Make sure to have 'ip nat inside' and 'ip nat outside' under the correct interface. 
  • Make sure you are seeing hits. 
  • Make sure you have the correct 'ip nat inside source' statement with the correct access-list. 


Many more articles to come so stay tuned. If you like what you read please comment/like/+1/join this site --->