To help extend the life of the IPv4 addressing scheme while the newer IPv6 protocol is developed and deployed, other technologies have been developed. One of the most important of these is IP Network Address Translation (NAT). This technology allows a small number of public IP addresses to be shared by a large number of hosts using private addresses. This essential little “trick” allows the global Internet to actually have far more hosts on it than its address space would normally support. At the same time, it provides some security benefits by making hosts more difficult to address directly by foreign machines on the public Internet. In this document, we are going to brief IP NAT Overview, Motivation, Advantages and Disadvantages.

The decision to make IP addresses only 32 bits long as part of the original design of the Internet Protocol led to a serious problem when the Internet exploded in popularity beyond anyone’s expectations: exhaustion of the address space. Classless addressing helped make better use of the address space, and IPv6 was created to ensure that we will never run out of addresses again. However, classless addressing has only slowed the consumption of the IPv4 address space, and IPv6 has taken years to develop and will require years more to deploy.

The shortage of IP addresses promised to grow critical by the end of the 1990s unless some sort of solution was implemented until the transition to IPv6 was completed. Creative engineers on the Internet Engineering Task Force (IETF) were up to the challenge. They created a technique that would not only forestall the depletion of the address space but could also be used to address two other growing issues in the mid-to-late 1990s:

 

One solution to the problems of IP address space and security was to set up a system where a company’s network was not connected directly to the Internet, but rather indirectly. Setting up a network this way is possible due to several important characteristics of how most organizations use the Internet:

The best way to explain why these attributes matter is to draw an analogy to how telephones have used an organization because many of the same attributes apply there. Most of the telephones in a typical organization are used to let employees make phone calls out. Usually there is no need to have any way to call employees directly; instead, one system or person can handle all incoming calls. Only a few employees are ever making a call to the “outside world” at any given time. And all calls are routed through a central point that manages the telephone system.

For these reasons, to save money, organizations don’t run separate public telephone lines to every employee’s desk. Instead, it sets up a telephone system where each employee gets an extension, which is basically a local telephone number valid only within the organization. A small number of outside lines is made available in a pool for employees to share, and the telephone system matches the inside extensions to the outside lines as needed. A voice mail system and/or human receptionist handle routing of calls into the organization.

A very similar technique can be used for connecting an organization’s computers to the Internet. In TCP/IP networks, this technology was first formalized in RFC 1631, The IP Network Address Translator (NAT), adopted in May 1994. The word “translator” refers to the device (router) that implements NAT. More commonly, the technology as a whole is called IP Network Address Translation (IP NAT or NAT).

A basic implementation of NAT involves setting up an organization’s internal network using one of the private addressing ranges set aside for local IP networks. One or more public (Internet) addresses are also assigned to the organization as well, and one or more NAT-capable routers are installed between the local network and the public Internet. The public IP addresses are like “outside lines” in the telephone system, and the private addresses are like “internal extensions”.

The NAT router plays the role of telephone system computer and receptionist. It maps internal extensions to outside lines as needed, and also handles “incoming calls” when required. It does this by not just routing IP datagrams but modifying them as needed, translating addresses in datagrams from the private network into public addresses for transmission on the Internet, and back again.

Over time, newer versions of NAT have also been created that solve other problems or provide additional capabilities. Port-Based NAT allows sharing of even more hosts on a limited number of IP addresses, by letting two or more devices share one IP address at a time. So-called “Twice NAT” helps with the implementation of virtual private networks (VPN) by translating both source and destination addresses in both incoming and outgoing datagrams.

NAT is one of those technologies that have a long list of both advantages and disadvantages. This means it can be extremely useful in a variety of scenarios, but also problematic in others. The main advantages are:

The above are all good reasons to use NAT, but there are drawbacks to the technique as well. Some of these take away part of the benefit in certain items in the list above:

 

Many organizations feel that the advantages outweigh the disadvantages, especially if they do use the Internet in primarily a client/server fashion, as most do. For this reason, NAT has become quite popular. One should always bear in mind, however, that the main problem that led to NAT is lack of address space. IPv6 fixes this problem, while NAT merely finds a clever “workaround” for it. For this reason, many people consider NAT a “kludge”. Once IPv6 is deployed, it will no longer be needed, and some folks don’t like it even for IPv4. On the other hand, some feel its other benefits make it worthy of consideration even in IPv6.