What Is Address Translation - THOMSON SpeedTouch Configuration Manual

What is address translation

1.2 What is address translation
Introduction
Using address translation, the IP and/or TCP/UDP port identifications can change
while traversing the network.
IP address translation
SpeedTouchTM
Address
Translation
Private Public
Figure 1: IP address translation
Quite some private networks use private IP addresses, meaning that these host are
not known (and as such cannot be routed) within the Internet (= public network).
When such a host would enter the Internet, one should assign it an IP identification,
which is allowed to be used in the public network.
This is what will be done. The private network will have an exit point towards the
public network (typically called a gateway) and this exit point will convert the IP
address of the private network towards a valid public IP address. The reverse
procedure is followed when a packet is received back from the public network.
As the address translation is only performed on-demand and due to the fact that not
all private hosts need access to the public network at the same time, a small pool of
available public IP addresses on the WAN interface of the modem will be sufficient,
this means sparing out addresses.
TCP/UDP port
This type of translation will commonly be used in conjunction with IP address
translation translation. Not only the IP address will be changed but also the port number.
The main advantage of this way of working is that the same public IP address can be
assigned to two different private LAN nodes, yet using a different port numbering
scheme, which in fact boils down to IP address multiplexing.
As several nodes can use the same IP address, a considerable saving is done on the
amount of public IP addresses needed.
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