User's Manual

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7.3.2 How NAT Works
Each packet has two addresses – a source address and a destination address. For outgoing
packets, the ILA is the source address on the LAN, and the IGA 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 ROUTER
keeps track of the original addresses and port numbers so incoming reply packets can have
their original values restored.
The following figure illustrates this.