NAT
Network
Address Translation
NAT is the
process of altering the IP header of a packet so that the destination address,
the source address, or both addresses are replaced in the header by different
addresses. This swapping process is performed by a device running specialized
NAT software or hardware.
A NAT
enabled device typically operates at the border of a stub domain. A stub domain
is a network that has a single connection to the outside world. When a host
inside the stub domain, such as 10.1.1.6, wants to transmit to a host on the
outside, it forwards the packet to its default gateway. In this case, the
host's default gateway is also the NAT box.
The NAT
process running on the router looks inside the IP header and, if appropriate,
replaces the local IP address with a globally unique IP address.
When an outside host sends a response, the NAT router receives it,
checks the current table of network address translations, and replaces the
destination address with the original inside source address. NAT provides
tremendous benefits to individual companies and the Internet as well. Before
NAT, a host with a private address could not access the Internet. With NAT,
individual companies can address some or all of their hosts with private
addresses and then use NAT to access the public Internet. At the same time,
these hosts connect to the Internet without necessarily depleting its address
space.
NAT has several advantages, including the following:
Le NAT présente un avantage du point
de vue de la sécurité. En effet aucun équipement n’est visible depuis
l’extérieur du réseau. Tous les paquets sont adressés au routeur qui grâce à sa
table redirige les paquets vers le destinataire.