ZyXEL Communications PMG5318-B20A - Page 72

Chapter 7 NAT
PMG5318-B20A User’s Guide
72
NAT never changes the IP address (either local or global) of an outside host.
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 in each packet and then forwards it to the Internet. The
GPON 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 38 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.
Table 26 NAT Definitions (continued)
ITEM DESCRIPTION
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
Internet
WAN
LAN
Inside Local
Address (ILA)
Inside Global
Address (IGA)