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NAT and PAT - What's the Difference?


By Brian Scheibe

Network Address Translation (NAT) and Port Address Translation (PAT) both map IP addresses on an internal network to IP addresses on an external network. Which method of address translation you use depends on the types of networks that you are translating and the number of available IP addresses that you have.

If you are connecting a site in the network to a site in the network, you could use NAT to translate IP addresses to available IP addresses so that hosts on the network can access data and use network resources on the network. However, for this scenario to work, you must have an address pool that contains enough available IP addresses on the network to accommodate every host on the network, because NAT requires a one-to-one relationship when translating IP addresses.

PAT attempts to use the original source port number of the internal host to form a unique, registered IP address and port number combination. For example, two hosts that have been assigned the IP addresses and, respectively, could send traffic to and receive traffic from the Internet by using the single public IP address If that port number is already allocated, PAT searches for an available alternate source port number. Therefore, the host at IP address could access the Internet by using the public IP address and source port combination of Meanwhile, the host at IP address could access the Internet by using the IP address and source port combination of

If you are connecting a site in the network to the Internet, you must translate host IPs on that network to a registered IP address that is routable over the Internet. In order to use traditional NAT in this scenario, you would need to purchase a registered IP address for each host on your internal network. Alternatively, you could use PAT to translate all the IP addresses on the internal network to a single, shared IP address that connects to the Internet. PAT, which is also known as NAT overloading, uses 16-bit source port numbers to map and track traffic between an internal host and the Internet.

As you can see, the first letter in each acronym denotes the difference between NAT (Network Address Translation) and PAT (Port Address Translation), which should make it easier for you to remember which does what. Just remember that both NAT and PAT use at least one IP address and that PAT is also referred to as NAT overloading because it uses one IP address for all clients to multiple ports, whereas standard NAT uses a one-to-one IP address relationship per client.


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Good concept
Posted @ Thursday, August 22, 2013 8:21 AM by Raju Sinha
Bay far the best explaination of traditional NAT. Many people don't realize that for 1:1 NAT to work they require as many public IP's as there are for private on the local LAN/LAN's. 
Thumbs Up to this author!
Posted @ Wednesday, May 07, 2014 1:20 AM by Chris Oelke
Very good information!
Posted @ Friday, May 09, 2014 3:25 PM by Alberto Banegas
Excellent, well rounded explanation...well done.
Posted @ Sunday, June 22, 2014 1:39 AM by Luke C
Well in this case then the NAT is not so much useful, because if we purchase one public IP address for each of our clients and then give it to NAT to translate it, why not instead of using NAT we just give the purchased public IP address to every client since we are parchasing one public IP address for NAT to translate?
Posted @ Saturday, February 21, 2015 8:35 PM by Hashmatullah Azizi
Thank you for your comments. If you purchased one public IP address you would have to use PAT if you have more than one device in your network that needs to be accessible from external networks. If you purchase a public IP address for each device you would not need NAT or PAT for the devices to access external networks. The purpose of NAT/PAT is to allow you to use fewer public IP addresses in your network thus reducing the cost and conserving public IP address usage.  
Posted @ Monday, February 23, 2015 8:30 AM by Kelson Lawrence
Thank you that helped.
Posted @ Monday, March 16, 2015 9:07 AM by Marco
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