ZyXEL Communications VSG1432-B101 Series Manual De Usuario

 Chapter 12 Network Address Translation (NAT)

VSG1432-B101 Series 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 ZyXEL 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.

Figure 73   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)