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| 6.1 |  | Characteristics of Transport Layer Technologies |
| 6.1.6 |  | Static IP Network Address Translation |

‘The IP Network Address Translator’ is a technology defined in RFC 1631 that allows one group of addresses to be represented by another group of addresses. NAT is technically a network layer technology, but has some features that extend into the transport layer.

As IPv4 started to run out of addresses, several measures were developed to alleviate the pressure. One of these measures was the introduction of reserved private network address spaces in RFC 1918. This RFC reserves three address spaces which are not recognized or routed by the Internet and can be re-used by networks which do not need to connect to other networks or to the Internet. Internet core routers are configured to drop any packets either sourced from or destined to an address in any of these reserved address spaces. The three address spaces reserved by RFC 1918 are shown in Figure .

NAT is a process which allows networks originally configured with one of these reserved address spaces to connect to other privately addressed networks and to the Internet without having to re-address the internal network. NAT is normally an additional process that is run on routers operating on the boundary between two discrete networks.

NAT works, either statically or dynamically, by using a table of IP addresses to re-write the addressing information in an IP packet header. In its simplest form, the network engineer manually builds the NAT table. The table has addresses used inside the network mapped to addresses used outside the network. Each table entry represents a single host inside the network.

The following process shows the operation of NAT:

- User at host 10.4.1.1 opens a connection to host B.
- The first packet that the router receives from 10.4.1.1 causes the router to check its NAT table. A translation is found because it has been statically configured.
- The router replaces the inside local IP address 10.4.1.1 with the selected inside global address (2.2.2.2) and forwards the packet.
- Host B receives the packet and responds to 10.4.1.1 using the inside global IP address 2.2.2.2.
- When the router receives the packet with the inside global IP address of 2.2.2.2, the router performs a NAT table lookup using the inside global address as the reference. The router then translates the address back to 10.4.1.1 and forwards the packet to the host.
- The host with the 10.4.1.1 address receives the packet and continues the conversation.