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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,
your BIPAC 5100S 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).
6.1.3 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 BIPAC 5100S keeps track of the original addresses and
port numbers so incoming reply packets can have their original values restored. The following
figure illustrates this.
6.1.4 NAT Application
The following figure illustrates a possible NAT application, where three inside LANs (logical
LANs using IP Alias) behind the BIPAC 5100S can communicate with three distinct WAN
networks. More examples follow at the end of this chapter.