Black Box Version 1.0 NAT functioning, Static Network Address Translation, Masquerading NAPT, PAT

If there is a necessity to divide traffic on the basis of TCP ports, NAT makes it possible to map local addresses with one external address using TCP load distribution function.

NAT functioning

The NAT technology defines, as it is stated in the RFC 1631 standard, the ways of IP address translation, used in one network into another network addresses.

There exist three basic principles of address translation: static, dynamic and masquerading.

Static Network Address Translation

With the help of this concept, NAT can organize translation between the same class networks. (For example, when each of two networks contain one address (mask – 255.255.255.255). This strategy is the simplest, because the translation can be described by a couple of simple logical transformations.

Let us cite an example of address translation from two Class C networks – 194.24.90 and

195.60.3.While passing through NAT to the sender’s address field, the packet, addressed from the host 194.24.90.13 will contain a change in the IP header from 194.24.90.13 to 195.60.3.13.

Dynamic Address Translation

Dynamic translation is necessary when the number of addresses (internal and external) being translated is different, however, dynamic translation is sometimes used when static translation does not work. The number of intercommunicating hosts will be limited, in any case, by the number of free (available) addresses on the NAT interface.

Dynamic NAT is more complicated, because it requires to keep track of intercommunicating hosts and possibly even of connections, in case when the information (content) must be modified at Layer 4 (TCP, for example).

For example it is necessary to translate dynamically all IP addresses in Class B network 138.201 into addresses of Class C network 190.200.112. Then, each new connection receives an address from Class C network if there are available addresses there.

This technology, in contrast with static translation, introduces a new notion – NAT table. It is a rendition table of internal addresses and NAT-interface addresses (hereinafter, NAT addresses)

Masquerading (NAPT, PAT)

The Port Address Translation is another case of dynamic translation. Here, we have only one external address behind which, internal addresses “are hidden” – there can be as many internal addresses as possible. In contrast to the original dynamic translation, PAT does not mean that there can be only one connection at a time. To multiplex the number of connections, TCP port information is used by this masquerading. Thus, only the number of ports available limits number of simultaneous connections.

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