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Network Address Translation NAT Tutorial

May 22nd, 2011 Go to comments

To go to the Internet we need to get an public IP address and it is unique all over the world. If each host in the world required a unique public IP address, we would have run out of IP address years ago. But by using Network Address Translation (NAT) we can save tons of IP addresses for later uses. We can understand NAT like this:

“NAT allows a host that does not have a valid registered IP address to communicate with other hosts through the Internet”

For example your computer is assigned a private IP address of and of course this address can not be routed on the internet but you can still access the internet. This is because your router (or modem) translates this address into a public IP address, for example, before routing your data into the internet.


Of course when your router receives a reply packet destined for it will convert back to your private IP before sending that packet to you.

Maybe you will ask “hey, I don’t see any difference of using NAT to save tons of IP addresses because you still need a public IP address for each host to access the Internet and it doesn’t save you anything, why you need to use NAT?”

Ok, you are right :), in the above example we don’t see its usefulness but you now understand the fundamental of NAT!

Let’s take another example!

Suppose your company has 500 employees but your Internet Service Provider (ISP) only gives you 50 public IP addresses. It means that you can only allow 50 hosts to access the internet at the same time. Here NAT comes to save your life!

One thing you should notice that in real life, not all of your employees uses internet at the same time. Say, maybe 50 of them use internet to read newspaper at the morning; 50 others use internet at noon for checking mail… By using NAT you can dynamically assign these 50 public IP addresses to those who really need them at that time. This is called dynamic NAT.

But the above NAT solution does not solve our problem completely because in some days there can be more than 50 people surfing web at the morning. In this case, only the first 50 people can access internet, others must wait to their turns.

Another problem is, in fact, your ISP only gives you much lesser IP addresses than the number 50 because each public IP is very precious now.

To solve the two problems above, another feature of NAT can be used: NAT Overload or sometimes called Port Address Translation (PAT)

PAT permits multiple devices on a local area network (LAN) to be mapped to a single public IP address with different port numbers. Therefore, it’s also known as port address translation (PAT). When using PAT, the router maintains unique source port numbers on the inside global IP address to distinguish between translations. In the below example, each host is assigned to the same public IP address 1 but with different port numbers (from 1000 to 1002).


Note: Cisco uses the term inside local for the private IP addresses and inside global for the public IP addresses replaced by the router.

The outside host IP address can also be changed with NAT. The outside global address represents the outside host with a public IP address that can be used for routing in the public Internet.

The last term, outside local address, is a private address of an external device as it is referred to by devices on its local network. You can understand outside local address as the inside local address of the external device which lies at the other end of the Internet.

Maybe you will ask how many ports can we use for each IP? Well, because the port number field has 16 bits, PAT can support about 216 ports, which is more than 64,000 connections using one public IP address.

Now you has learned all the most useful features of NAT but we should summary all features of NAT:

There are two types of NAT translation: dynamic and static.

Static NAT: Designed to allow one-to-one mapping between local and global addresses. This flavor requires you to have one real Internet IP address for every host on your network.

Dynamic NAT: Designed to map an unregistered IP address to a registered IP address from a pool of registered IP addresses. You don’t have to statically configure your router to map an inside to an outside address as in static NAT, but you do have to have enough real IP addresses for everyone who wants to send packets through the Internet. With dynamic NAT, you can configure the NAT router with more IP addresses in the inside local address list than in the inside global address pool. When being defined in the inside global address pool, the router allocates registered public IP addresses from the pool until all are allocated. If all the public IP addresses are already allocated, the router discards the packet that requires a public IP address.

PAT (NAT Overloading): is also a kind of dynamic NAT that maps multiple private IP addresses to a single public IP address (many-to-one) by using different ports. Static NAT and Dynamic NAT both require a one-to-one mapping from the inside local to the inside global address. By using PAT, you can have thousands of users connect to the Internet using only one real global IP address. PAT is the technology that helps us not run out of public IP address on the Internet. This is the most popular type of NAT.

Besides NAT gives you the option to advertise only a single address for your entire network to the outside world. Doing this effectively hides the internal network from the public world really well, giving you some additional security for your network.

NAT terms:

* Inside local address – The IP address assigned to a host on the inside network. The address is usually not an IP address assigned by the Internet Network Information Center (InterNIC) or service provider. This address is likely to be an RFC 1918 private address.
* Inside global address – A legitimate IP address assigned by the InterNIC or service provider that represents one or more inside local IP addresses to the outside world.
* Outside local address – The IP address of an outside host as it is known to the hosts on the inside network.
* Outside global address – The IP address assigned to a host on the outside network. The owner of the host assigns this address.


To learn how to configure NAT please read my Configure NAT GNS3 Lab tutorial

Comments (50) Comments
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1 2 3 6 511
  1. zahid
    May 26th, 2011

    thanks for guiding such a great manner

  2. lelo
    May 26th, 2011

    tomorrow is my day

  3. lelo
    May 26th, 2011

    thanks 9tut, great job

  4. lelo
    May 27th, 2011

    score 894

  5. netcert
    May 29th, 2011

    in addition to this here is a video that explains and configures NAT in detail

  6. RAJ
    June 9th, 2011

    exactly how many devices can br PAT-ed by a single router ??

  7. ReR
    June 28th, 2011

    great info great job!! many thanxxx

  8. jetingarg
    July 14th, 2011

    Thanks a lottttttttttttttttttt

  9. Anonymous
    July 16th, 2011

    Hey netcert, the link to the video that you have given was really very useful. It explains NAT in a very detailed manner.

  10. Paulie
    July 21st, 2011

    I’m I dont know where to begin, am really grateful for the support…thank you 9tut

  11. panchavarshik yojna
    July 28th, 2011

    thanks a lot for the clear explanation great help

  12. Revathy Rams
    August 10th, 2011

    Thanks a lotttttttttttttttt for the clear explanation

  13. Anon
    August 12th, 2011

    This site is so awesome, props!

  14. ndawi
    August 15th, 2011

    this is it,
    thanks very much for clear elaboration .

  15. joyesh
    August 24th, 2011

    this is thank you very much

  16. shabani rajabu
    August 27th, 2011

    greet Job

  17. vivek
    September 18th, 2011

    thanks a lot for very clear describsation.

  18. ermias
    September 26th, 2011

    again tanks for exploration.

  19. varun
    November 5th, 2011

    thanks alot

  20. MSR
    November 18th, 2011

    i am beginner in networking

    pls explain about nat

    inside local address

    inside global address

    outside local address

    outside global address


  21. xallax
    November 18th, 2011

    have you read the tutorial above? please do so. thank you

  22. MSR
    November 20th, 2011


    i was read !but these confusing!

    just explain!

  23. abdou
    November 21st, 2011

    thanks to 9tut really i understood nat and pat , please keep helping us .

  24. aJmAL
    December 4th, 2011

    very nice teach about nat

  25. Anonymous
    December 15th, 2011

    Really very good tutorials……….

  26. Avinash Balasaria
    December 15th, 2011

    Really very good tutorials……….
    and they are very helpfull explaning teh concepts very properly and clearly

  27. barry
    December 16th, 2011

    @MSR, if you don’t understand the tutorial above, plz jump in the sea and never come back here hahaha

  28. Claudio
    December 21st, 2011

    MSR check the video that netcert suggest. it s very clear. I was lost until watch this.

  29. jerry
    December 29th, 2011

    thanks for the elaboration

  30. dror
    January 19th, 2012

    i love cisco

  31. Faheem
    January 20th, 2012

    I have read whole chapters of NAT but was not as crystal clear as i am now after reading this article… many thanks great job 9tut

  32. islam
    January 20th, 2012

    what does ” prefix-length “19 mean ????

  33. Sam
    January 31st, 2012

    what is the syntax for configuring diferent nat types. comman syntax missing in this tutorial.

  34. amoffPoemyGor
    February 2nd, 2012

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  35. Jefss
    February 26th, 2012

    What about the comands????

  36. Eli
    February 29th, 2012

    you write here that PAT is many-to-one, but it is also many to few. isn’t it?

  37. Cezar
    February 29th, 2012


  38. timoty
    March 3rd, 2012

    wonderful website for those who face ccna test.

  39. ekta vinda
    March 7th, 2012

    thanks for this wonderful information of NAT.

  40. ekta vinda
    March 7th, 2012

    thanks, 4 helping this explanation of NAT with demo.

  41. juliet
    March 10th, 2012

    thanks 9tut 4 this tutorial.its mind blowing

  42. vijayakumar
    March 18th, 2012

    very nice information

  43. firebee
    March 23rd, 2012

    Thank so much

  44. morris
    March 24th, 2012

    Always 9tut the best snapshot to CCNA. Many thanks.. simple and straight

  45. thanks 9tut
    March 27th, 2012

    i hope i will pass my exam the day after tomorow

  46. Ben
    March 28th, 2012

    really nice tutorial, WAY TO GO 9TUT!!

  47. von
    March 30th, 2012

    thanks for the materials , this is marvelous

  48. vijay suthar
    April 3rd, 2012

    thank you 9tut, i love u.uuuuuuuuuuuuuuuuuu

  49. vijay suthar
    April 14th, 2012

    really great explanation…

  50. kaathi
    April 18th, 2012

    thank u … this is awesome example

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