NAT (Network Address Translation)

With NAT, not only Private to Public IP Address translation is used. Beside, Public to Puclic or Private to Private translations are also done via NAT. But the first one is most common.

If you would like to test yourself, you can check CCNA Quizes Page.

NAT Types

Static NAT

Static NAT is the type of NAT that is used for One-to-One Translation of Ports or IP Addresses. In other words, for example in this NAT type, one Private IP Address is mapped to one Public IP Address.

In Static NAT, if you do your firewall setting well, then a Public IP can access to your Internal user. Because, here, IP translation is being done One-to-One.

Below, there is an example of Static NAT. In this Static NAT Example, each Private IP Address is translated to a specific Public IP Address. For example, 192.168.0.1 is translated to 200.200.200.1.

Dynamic NAT

Dynamic NAT is the NAT type that is used with a Public IP Address Pool and works with more than one Public IP Address. Here, multiple Private IP Addresses are mapped to a Pool of Public IP Addresses.And these IP Addresses are given to the Internal users randomly. So, it is difficult to reach any Internal user from outside.

Dynamic NAT is used when the number of internal Internet users are known.

Below, there is an example of Dynamic NAT. In this Dynamic NAT Example, each Private IP Address is translated to a specific Public IP Address in Public IP Pool. The translation is random. For example, 192.168.0.3 is translated to 200.200.200.7.

PAT (NAT Overload)

PAT (Port Address Translation) is one of the NAT types that is also known as NAT Overload. Here, many Private IP Addresses are translated to one Public IP Address. The traffic distinguisher in PAT are Port Numbers,  TCP/UDP ports are used in PAT (NAT Overload).

Each IP Address’s traffic is determined by these ports. If you have many Intrenet user in a location, this type of NAT is very useful for you.

Below, there is an example of PAt (NAT Overload). In this PAT Example, Private IP Addresses are translated to a specific Public IP Address with the help of Port Numbers. For example, 192.168.0.4:21 is translated to 200.200.200.1:21.

NAT Key Terms

By the way, there are some terms that we must explain in this lesson about NAT (NAT Address Translation). These NAT terms are given below:

 

• Inside Local: Your PC’s private address
• Inside Global:Public address assigned to your PC
• Outside Local:Outside host’s public address
• Outside Global:Same address as the outside local, necessary to translate an outside address to a private address.

 

Let’s explain these NAT terms with an example.

 

NAT TermsAt the above example, there are two hosts, two routers and a server. Here, we will explain NAT terms from host A’s perspective. Host A will reach Server C and Host B over internet. And on Router A, NAT is enabled.

 

Host A has a private ip address, 10.1.1.10 and this is our Inside Local Address. With this address Host A can not rach to Internet. So, it uses Ruter A’s public ip address, 55.1.1.1 and this is our Inside Global Address.

 

There is a Router B and Server C on the above screen. The interface ip addresses of these devices are Outside Global Addresses. These addresses are 88.1.1.1 and 99.1.1.1. Host B has a private ip address that is used in its local network. But Host A sees its global ip address as Outside Local Address. So, our ouside local address is also 99.1.1.1.

 

As a summary our nat terms will be like below:

• Inside Local Address– 10.1.1.10
• Inside Global Address– 55.1.1.1
• Outside Global Address– 88.1.1.1, 99.1.1.1
• Outside Local Address– 99.1.1.1

 

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