ZyXEL Communications PMG5318-B20A User Manual page 72

Chapter 7 NAT
Table 26 NAT Definitions (continued)
ITEM
Local
Global
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
LAN
192.168.1.12
192.168.1.11
72
DESCRIPTION
This refers to the packet address (source or destination) as the packet travels on the LAN.
This refers to the packet address (source or destination) as the packet travels on the WAN.
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)
WAN
Internet
PMG5318-B20A User's Guide