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Talking IP: The challenge of NAT in IPv6 (1)

By Segun Sorunke
The Internet has grown larger than anyone ever imagined it could be! Although the exact size is unknown, the current estimate is that there are about 150 million hosts and over 500 million users.

When IP addressing first came out, everyone thought there were plenty of addresses to cover any need; theoretically, you could have 4,294,967,296 unique addresses . However, the actual number of available addresses is smaller because of the way that the addresses are separated into Classes and the need to set aside some of the addresses for multicasting, testing or other specific uses.

One of the most often-stated ‘justifications’ for IPv6 is the issue of IPv4 address exhaustion; with the unprecedented expansion of Internet usage in recent years – especially by population dense countries like India and China – the impending shortage of address space (availability) was recognized by 1992 as a serious limiting factor to the continued usage of the Internet run on IPv4.

With the explosion of the Internet and the increase in home and business networks, the number of available IP addresses is simply not enough, and this is one of the main reasons for the development of IPv6, which will give greater address space. IPv6 supports addresses that are four times the number of bits as IPv4 addresses (128 vs. 32). This works out to be 340, 282, 366, 920, 938, 463, 463, 374, 607, 431, 768, 211, 456.

Network Address Translation (NAT) was not the only option that was designed to address IP address shortage; another popular scheme is Classless InterDomain Routing (CIDR). A CIDR address is still a 32-bit address, but it is hierarchical rather than class-based. NAT was developed specifically to address IP address shortage in particular instances when the cost of extra IP address is an issue.

NAT is therefore of particular interest in countries other than the United States where historically there have been fewer addresses allocated per capita; and also in small businesses and home offices.

While NAT came into being in the 1990’sas a popular tool for alleviating the IPv4 address exhaustion problem, it eventually became an indispensable tool for most home and small-business networks.NAT is a Cisco version of Port Address Translation (PAT), and enables a LAN to use one set of IP address for internal traffic and a second set of address for external traffic. This allows a company to shield internal addresses from the public Internet.

NAT is used by a device
(firewall, router, or computer) that sits between an internal network and the rest of the world. There are TWO main types of NAT : dynamic and static. In static NAT, the public IP address is always the same, allowing an internal host, such as a Web server, to have an unregistered private IP address and still be reached over the Internet.

In dynamic NAT, a private IP address is mapped to a public IP address drawn from a pool of registered public IP addresses. By keeping the internal configuration of the private network hidden, dynamic NAT helps conceal the network from outside users.

NAT is sometimes confused with proxy servers but there are definite differences. NAT is transparent to the source and destination computers. Neither one realises that it is dealing with a third device. But a proxy server is not transparent. The source computer knows that it is making a request to the proxy server and must be configured to do so.

The destination computer thinks that the proxy server IS the source computer and deals with it directly. Also, proxy servers usually work at Layer 4 (transport) of the OSI Reference Model, while NAT operates at Layer 3 (network). Working at a higher layer makes proxy servers slower than NAT devices in most cases.

The usage of NAT also carries certain drawbacks:
lNetwork Address Translation does not allow a true end-to-end connectivity that is required by some real time applications.

A number of real-time applications require the creation of a logical tunnel to exchange the data packets quickly in real-time. It requires a fast and seamless connectivity devoid of any intermediaries such as a proxy server that tends to complicate and slow down the communications process.

lNAT creates complications in the functioning of Tunnelling protocols. Any communication that is routed through a Proxy server tends to be comparatively slow and prone to disruptions. Certain critical applications offer no room for such inadequacies.

Examples include telemedicine and teleconferencing. Such applications find the process of network address translation as a bottleneck in the communication network creating avoidable distortions in the end-to-end connectivity.

lNAT acts as a redundant channel in the online communication over the Internet. The twin reasons for the widespread popularity and subsequent adoption of the network address translation process were a shortage of IPv4 address space and the security concerns. Both these issues have been fully addressed in the IPv6 protocol.

As the IPv6 slowly replaces the IPv4 protocol, the network address translation process will become redundant and useless while consuming the scarce network resources for providing services that will be no longer required over the IPv6 networks.
lContinues next week.


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