How Nat Works - ZyXEL Communications VMG1312-B Series User Manual

Chapter 11 Network Address Translation (NAT)

11.8.3 How NAT Works

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.
Figure 84 How NAT Works
LAN
192.168.1.12
192.168.1.11
196
192.168.1.13
SA
192.168.1.10
Inside Local
Address (ILA)
192.168.1.10
NAT Table
Inside Local Inside Global
IP Address
IP Address
192.168.1.10
IGA 1
192.168.1.11
IGA 2
192.168.1.12
IGA 3
192.168.1.13
IGA 4
SA
IGA1
Inside Global
Address (IGA)
VMG1312-B Series User's Guide
WAN