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Dynamic NAT - Part 1

Posted in Network Address Translation - NAT

Dynamic NAT is the second NAT mode we're going to talk about. Dynamic NAT, like Static NAT, is not that common in smaller networks but you'll find it used within larger corporations with complex networks.

The way Dynamic NAT differentiates from Static NAT is that where Static NAT provides a one-to-one internal to public static IP mapping, Dynamic NAT does the same but without making the mapping to the public IP static and usually uses a group of available public IPs.


What Exactly Does Dynamic NAT do?

While looking at Static NAT, we understood that for every private IP Address that needs access to the Internet we would require one static public IP Address. This public IP Address is mapped to our internal host's IP Address and it is then able to communicate with the rest of the world.

With Dynamic NAT, we also map our internal IP Addresses to real public IP Addresses, but the mapping is not static, meaning that for each session our internal hosts communicate with the Internet, their public IP Addresses remain the same, but are likely to change. These IPs are taken from a pool of public IP Addresses that have been reserved by our ISP for our public network.

With Dynamic NAT, translations don't exist in the NAT table until the router receives traffic that requires translation. Dynamic translations have a timeout period after which they are purged from the translation table, thus making them available for other internal hosts.

The diagram below illustrates the way Dynamic NAT works:


Static NAT - Part 2

Posted in Network Address Translation - NAT

The previous page helped us understand what exactly happens with Static NAT and how it works, and we saw a few examples of how to use it in various network configurations.

This page will deal with the transformations the packets undertake as they pass through the Static NAT device, which is normally a router or firewall appliance.

So let's get started ! Now would be a good time to fill that cup of yours and reload yourself with your special edible supplies :)


How NAT Translations Take Place

So what exactly happens to the packet that enters or exits the Static NAT-enabled device ? Well it's not that complicated once you get the hang of it. The concept is simple and we're going to see it and analyse it using an example, which is really the best possible approach.

The process of the Static NAT translation is the same for every device that supports it (assuming the manufacturer has followed the RFCs). This means that whether we use a router or a firewall appliance to perform Static NAT they'll both follow the same guidelines.

Consider our example network:



As the diagram describes we have Workstation No.1, which sends a request to the Internet. Its gateway is the router that connects the LAN to the Internet and also performs Static NAT.

The diagram below shows us how the Workstation's packet is altered as it transits the router before it's sent to the Internet (outgoing packet):



As you can see, the only thing that changes is the Source IP, which was and was given the value of, which is a real IP Address on the Internet. The Destination IP Address, Source Port and Destination Port are not modified.

Assuming the packet arrives at its destination, we would most likely expect to see a reply. It would be logical to assume that the reply, or incoming packet, will require some sort of modification in order to successfully arrive at the originating host located on our private network (that's Workstation 1).

Here is how the incoming packet is altered as it transits the router:



The diagram above shows the part of the incoming packet that is altered by the router. Only the destination IP Address is changed, from to so the packet can then be routed to the internal workstation. Source IP Address, Source Port and Destination Port remain the same.

And in case you're wondering why the ports have changed in comparison to the original outgoing packet, this is not because of NAT but the way IP communications work and happens to be way out of the scope of this topic.

Now, because I understand that even a simple diagram can be very confusing, here's one more that summarises all the above. The diagram below shows you what the outgoing and incoming packets looked like before and after transiting the router:



So there you have it, Static NAT should now make sense to you :)

As you've seen, the concept is very simple and it varies slightly depending on the NAT mode you're working with. So NAT is not that difficult to understand after all ! If there are still a few things that are unclear to you, please try reading the page again and keep in mind the forum to which you can post your questions and doubts !

Next up is Dynamic NAT! So sit tight and let's rock and roll.... :)


Previous - Static NAT - Part 1                                                                                                                Next - Dynamic NAT - Part 1


Back to Network Address Translation Section

Static NAT - Part 1

Posted in Network Address Translation - NAT

Static NAT (also called inbound mapping) is the first mode we're going to talk about and also happens to be the most uncommon between smaller networks.

Static NAT was mainly created to allow hosts on your private network to be direcly accessible via the Internet using real public IPs; we'll see in great detail how this works and is maintained. Static NAT is also considered a bit dangerous because a misconfiguration to your firewall or other NAT-enabled device can result in the full exposure of the machine on your private network to which the public IP Address maps, and we'll see the security risks later on this page.


What Exactly Does Static NAT Do ?

As mentioned in the introduction, Static NAT allows the mapping of public IP Addresses to hosts inside the internal network. In simple english, this means you can have a computer on your private network that exists on the Internet with its own real IP.

The diagram below has been designed to help you understand exactly how Static NAT works:


The Network Address Translation Table

Posted in Network Address Translation - NAT

After that simple and informative introduction to the NAT concept, it's time to find out more about how it works and this is where the NAT table comes in.


The NAT Table

The NAT table is the heart of the whole NAT operation, which takes place within the router (or any NAT-enabled device) as packets arrive and leave its interfaces. Each connection from the internal (private) network to the external (public-Internet) network, and vice versa, is tracked and a special table is created to help the router determine what to do with all incoming packets on all of its interfaces; in our example there are two. This table, known as the NAT table, is populated gradually as connections are created across the router and once these connections are closed the entries are deleted, making room for new entries.

The NAT table works differently depending on the NAT mode. This is explained in greater detail on each NAT mode's page. For now, we just need to get the feeling for this table to facilitate understanding of each NAT mode.

The larger the NAT table (which means the more memory it occupies), the more bi-directional connections it can track. This means that a NAT-enabled device with a big NAT table is able to serve more clients on the internal network than other similar devices with smaller NAT tables.

The illustration below shows you a typical table of a NAT-enabled device while internal clients are trying access resources on the Internet:


Network Address Translation (NAT) Concepts

Posted in Network Address Translation - NAT

Before we dive into the deep waters of NAT, we need to make sure we understand exactly what NAT does. So let me give you the background of NAT, why it's here today and how it works. Even though there are different modes of NAT they are all basically extensions to the original concept.

NAT has become so popular that almost all small routers, firewall software and operating systems support at least one NAT mode. This shows how important it is to understand NAT.


The NAT Concept

NOTE: NAT is not only used for networks that connect to the Internet. You can use NAT even between private networks as we will see in the pages to follow, but because most networks use it for their Internet connection, we are focusing on that.

The NAT concept is simple: it allows a single device to act as an Internet gateway for internal LAN clients by translating the clients' internal network IP Addresses into the IP Address on the NAT-enabled gateway device.

In other words, NAT runs on the device that's connected to the Internet and hides the rest of your network from the public, thus making your whole network appear as one device (or computer, if you like) to the rest of the world.

NAT is transparent to your network, meaning all internal network devices are not required to be reconfigured in order to access the Internet. All that's required is to let your network devices know that the NAT device is the default gateway to the Internet.

NAT is secure since it hides your network from the Internet. All communications from your private network are handled by the NAT device, which will ensure all the appropriate translations are performed and provide a flawless connection between your devices and the Internet.

The diagram below illustrates this:



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