Natural Address Translation FAQ


By Art Reisman

Art Reisman CTO www.netequalizer.com

Editor’s note: Art Reisman is the CTO of APconnections. APconnections designs and manufactures the popular NetEqualizer bandwidth shaper.

Editors Note: The official term for one public IP address mapped to multiple private IP addresses is PAT. However, most IP people use the terms interchangeably.


I was doing some internal research on natural address translation (NAT) this past week, and as I looked for reliable sources, I became a bit frustrated with the information available. Yes, the information is out there and the Wikipedia article has some nice charts with all the details. But, if you’re looking  for the rational reasons behind NAT, you might want to shoot yourself in the head by the time you read through all of the information and find what you’re looking for.

To preserve your sanity, as well as answer some key questions quickly, I’ve put together the following Q&A detailing some key points when it comes to NAT. We’ll start with the basics and go from there.

What is NAT?

In order to allow multiple users to share a single IP address, modern routers utilize NAT to find unused port numbers and map them to a set of local private IP addresses. So, for example, let’s say your Internet provider gives you a single IP address for your household. It could be something like 98.245.90.60, which is a public IP address owned by Comast.

All of the computers in your house must share the single IP address that Comcast provides. So, your local router — the Linksys wireless router you bought for $79 — will use NAT to tag traffic with port numbers and then create some additional IP addresses right where your house connects to the Internet.

Let’s say you contacted the Microsoft website to download the latest service pack. When Microsoft sends you the download, it’s going to send it to 98.245.90.60:5001. “5001” is the port number established for the FTP transfer and 98.245.90.60 is the Comcast-owned Internet address for your entire house. Using NAT, your router will then interpret the port number and change the IP address to a unique internal address (like 192.168.1.103:8700, for example) before it gets to your computer.

Why do we need NAT?

NAT is useful because home users often have more than one computer in their household and yet only have a single IP address from their provider. Since every computer that talks on the Internet requires an IP address, it would not be possible to have more than one computer in your house without NAT.

How does NAT map a single IP address to multiple computers without things like Web browsing getting mixed up?

First, here’s some background on the difference between a base IP address and a port number. Internet addresses have two parts: an IP address, such as 98.243.90.60, and a port number. The IP address is used to route data across the Internet and the port is used by the receiving device — your computer — to determine what service to provide. For example, port 80 is the default port address for Web browsing.

Before the invention of NAT, Internet routers mostly ignored the port part of the address as they did not need it to move IP packets across the Internet. When describing the function of a port number, I like to use the analogy of a large dormitory with individual room numbers for the people living there. The postal service ignores the room numbers as their service ends at the address of the dormitory. They do not sort the mail by room number. For internet routers, port numbers are like room numbers. They deliver the packet to the end user’s computer and the port number is then interpreted.

The range of possible port numbers are in the tens of thousands, which is more than enough interpreting services by a user’s computer.  Think of a dorm with 1,000 residents in which they would only need 1,000 numbers for mailboxes, but still had 1,000,000 reserved.

What happens if there are no free ports to do the translation?

On small home networks this is not likely to happen, but you can get conflicts if, for example, you try to use NAT on a network with tens of thousands  of users. The total number of unique ports available is 65,000 and most users will require more than one port at a time.

Does NAT slow down my Internet connection?

Not enough for you to notice.

Why does my provider only allocate one IP address for my residence?

Even though there are about 4,000,000,000 (four billion) possible Internet addresses, the actual addresses are given out in large blocks, and once given out, they are hard to get back. So, and this is purely an example, let’s say a large company was given a class B set of addresses (which used to be common in the early days). They would have 64,000 addresses in their control. Hence, even with 4,000,000,000 possible addresses, they are in short supply, and your provider cannot afford to give them out more than one at a time.

Can I have more than one IP address?

Yes, but you would likely need a business class Internet service, which is generally quite a bit more expensive than residential-type service.

When will the world run out of IP addresses?

Some say we already have and there is a big push to go to a new standard called IPV6. However, we don’t think that will ever happen.

Editors Note: The official term for one public IP address mapped to multiple private IP addresses is PAT. However, most IP people use the terms interchangeably.

2 Responses to “Natural Address Translation FAQ”

  1. Bandwidth Control from the Public Side of a NAT Router, Is it Possible ? « NetEqualizer News Blog Says:

    […] basic problem with a generic bandwidth controller, is that they typically treat all users behind a NAT router as one […]


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