Atlantis Land 111U - Manual (Page 51)

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preventing intruders from probing your network. For more information on IP address
translation, refer to RFC 1631, The IP Network Address Translator (NAT).
8.1.3 How NAT Works
Each packet has two addresses – a source address and a destination address. For
outgoing packets, the ILA (Inside Local Address) is the source address on the LAN,
and the IGA (Inside Global Address) 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
ADSL 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.
8.1.4 NAT Application
The following figure illustrates a possible NAT application, where three inside LANs
(logical LANs using IP Alias) behind the ADSL Router can communicate with three
distinct WAN networks. More examples follow at the end of this chapter.