How Nat Works; Nat Application - Telkom ADSL 5100 Instruction Manual

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, your
TELKOM ADSL 5100 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 TELKOM ADSL 5100 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 TELKOM ADSL 5100 can communicate with three distinct WAN
networks. More examples follow at the end of this chapter.
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