ZyXEL SBG3300-NB00 User Guide - Page 236

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 85 How NAT Works LAN 192.168.1.13 192.168.1.12 SA 192.168.1.10 NAT Table 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 SA IGA1 WAN Inside Local Address (ILA) 192.168.1.11 192.168.1.10 Inside Global Address (IGA) 236 SBG3300-N Series User's Guide