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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.
6.3.3 NAT Application
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.