Mode com MC-420 - How NAT Works; NAT Application

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WIRELESS ROUTER ADSL

6.3.2 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 des-

tination address on the LAN, and the IGA is the destination address on the WAN. NAT maps private (local) IP address-

es 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 ﬁ gure illustrates this.

6.3.3 NAT Application

The following ﬁ gure 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.