Nat, Pat And Napt - THOMSON SpeedTouch Configuration Manual

2 NAT, PAT and NAPT

Introduction
NAT
PAT
E-NIT-CTC-20040716-0004 v1.0
Three different categories of address translation exist, namely:
Network Address Translation
public IP address Y.
Port Address Translation
port number Y.
Network Address and Port Translation
port number are translated.
If NAT is enabled, then a private IP address is changed into a temporary public IP
address. The NAT translation technique is often used in dial-up or for on-demand
connections in which remote connections go up and down frequently. When the user
is connected, s/he is assigned a single external IP address; once that user
disconnects, the IP address is released and becomes free for use again.
Private
192.168.0.1
SpeedTouch
`
192.168.0.1 192.168.0.254
HTTP to 30.0.0.1
Src : 192.168.0.1
Dst: 30.0.0.1
Src : 30.0.0.1
Dst: 192.168.0.1
As illustrated above, the SpeedTouch™ NAT box has an internally configured
mapping from the private IP address to a public one and vice versa. It is transparent
for NAT whether this table information is persistent or not.
Important to notice is that 30.0.0.1 thinks he receives a message from 20.0.0.1
instead of 192.168.0.1. So NAT hides the original originator.
PAT only changes the port number (TCP or UDP) of the packet. In most cases PAT is
used in combination with NAT. When NAT and PAT are used together, this is called
NAPT.
A common practise is that for outgoing packets the source port number is changed
and for incoming packets it will be the destination port.
PAT is also used when a service is not running on the default internal port. E.g. webs
er vice on port 8080 instead of 80. This will be explained later on in this document.
(NAT): a private IP address X is translated into a
(PAT): a UDP/TCP port number X is translated into a
(NAPT): both the private IP address and
Public
20.0.0.1
TM
20.0.0.1
Src : 20.0.0.1
Dst: 30.0.0.1
Src: 30.0.0.1
Dst: 20.0.0.1
Figure 2: NAT example
NAT, PAT and NAPT
Public
30.0.0.1
11