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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

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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Encore ENDSL-A2PLUS4R2 Manuale Utente Pagina 55