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A brief about PAT- Port Address Translation

Today I am going to talk about PAT- Port Address Translation or so called as NAT overloading. Before we start with the PAT, please check the facts of NAT in the below mentioned link which i explained earlier.


Well with the above mentioned article link, let me brief you about the NAT is short bullet points
  • Enables intra-networks that use private IP addresses to connect to the Internet by translating the address to a globally registered IP address.
  • Stores mapping of local to global address in NAT table
  • Increases network security by hiding internal IP addresses
  • Typically operates at the border of a stub network (single connection to neighbor network)
  • Private inside addresses = inside local
  • Public addresses = inside global

Now let's talk about the PAT- Port Address Translation, below are the points about PAT

Fig 1.1- PAT Sample Topology


  • NAT configured to advertise only one address for the entire internal network to the outside world “static PAT” or “address overloading” or “many-to-one”
  • Appends a unique source port number to each translation to outside IP address
  • Total number available internal addresses per 1 outside address is 65,536 ports
  • Attempts to assign first available port number, if already allocated assigns next number
Let's talk about the basic about how to configure the PAT- Port Address Translation 

Configure PAT (NAT overloading)

Step 1: Access list permits inside addresses to be translated 
access-list num permit inside-ip-net  wildcard-mask

Step 2: Define pool of inside global addresses (see dynamic NAT)
 ip nat pool pool-name  outside-start-ip  outside-end-ip net-mask subnet-mask  

Step 3: Establish overload translation to 
(a) specific interface (using interface IP address as outside address)
ip nat inside source list access-list-num interface outside-interface overload 

(b) specific pool (uses pool addresses as outside address)
ip nat inside source list access-list-num pool pool-name overload  

Step 4: Ethernet interface (inside local)
ip nat inside

Step 5: Serial interface (inside global)
ip nat outside

Verify PAT- Port Address Translation
  1. clear ip nat translation: clears all dynamic entries from NAT translation table
  2. clear ip nat translation inside: global-ip local-ip clears dynamic translation entry
  3. show ip nat translations: displays active translation
  4. show ip nat statistics: displays statistics

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