ZyXEL Communications Corporation VMG1312B10A Manuel D’Utilisation

Chapter 11 Network Address Translation (NAT)

VMG1312-B10A User’s Guide

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

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)