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before forwarding the packet to the WAN side. When the response comes back,
NAT translates the destination address (the inside global address) back to the
inside local address before forwarding it to the original inside host. Note that the
IP address (either local or global) of an outside host is never changed.
The global IP addresses for the inside hosts can be either static or dynamically
assigned by the ISP. In addition, you can designate servers, for example, a web
server and a telnet server, on your local network and make them accessible to the
outside world. With no servers defined, the ADSL Router filters out all incoming
inquiries, thus preventing intruders from probing your network. For more
information on IP address translation, refer to RFC 1631, The IP Network Address
Translator (NAT).
7.1.3 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.
7.1.4 NAT Application
The following figure illustrates a possible NAT application, where three inside
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