ZyXEL Communications Corporation AMG1302 Manuel D’Utilisation

 Chapter 9 Network Address Translation (NAT)

AMG1302-T10A User’s Guide

NAT never changes the IP address (either local or global) of an outside host.

In the simplest form, NAT changes the source IP address in a packet received from a subscriber 

(the inside local address) to another (the inside global address) before forwarding the packet to the 

WAN side. When the response comes back, NAT translates the destination address (the inside 

global address) back to the inside local address before forwarding it to the original inside host. Note 

that the IP address (either local or global) of an outside host is never changed.

The global IP addresses for the inside hosts can be either static or dynamically assigned by the ISP. 

In addition, you can designate servers, for example, a web server and a telnet server, on your local 

network and make them accessible to the outside world. If you do not define any servers (for Many-

to-One and Many-to-Many Overload mapping – see 

), NAT offers the 

additional benefit of firewall protection. With no servers defined, your Device filters out all incoming 

inquiries, thus preventing intruders from probing your network. For more information on IP address 

translation, refer to RFC 1631, The IP Network Address Translator (NAT).

Each packet has two addresses – a source address and a destination address. For outgoing packets, 

the ILA (Inside Local Address) is the source address on the LAN, and the IGA (Inside Global 

Address) is the source address on the WAN. For incoming packets, the ILA is the destination 

address on the LAN, and the IGA is the destination address on the WAN. NAT maps private (local) 

IP addresses to globally unique ones required for communication with hosts on other networks. It 

replaces the original IP source address (and TCP or UDP source port numbers for Many-to-One and 

Many-to-Many Overload NAT mapping) in each packet and then forwards it to the Internet. The 

Device keeps track of the original addresses and port numbers so incoming reply packets can have 

their original values restored. The following figure illustrates this.

How NAT Works

Local

This refers to the packet address (source or destination) as the packet travels on the LAN.

Global

This refers to the packet address (source or destination) as the packet travels on the WAN.

NAT Definitions (continued)

192.168.1.13

192.168.1.10

192.168.1.11

192.168.1.12

SA

192.168.1.10

SA

IGA1

Inside Local
IP Address

192.168.1.10
192.168.1.11
192.168.1.12
192.168.1.13

Inside Global 
IP Address
IGA 1
IGA 2
IGA 3
IGA 4

NAT Table

Inside Local
Address (ILA)

Inside Global
Address (IGA)