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.
7.3.3 NAT Application
54
The following figure illustrates a possible NAT application, where three inside LANs
(logical LANs using IP Alias) behind the router can communicate with three distinct WAN
networks. More examples follow at the end of this chapter.
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