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Chapter 10 Network Address Translation (NAT)
FMG3024-D10A / FMG3025-D10A Series User’s Guide
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address of the same inside host when the packet is on the WAN side. The following table
summarizes this information.
NAT never changes the IP address (either local or global) of an outside host.
10.4.2 What NAT Does
In the simplest form, NAT changes the source IP address in a packet received from a subscriber
(the inside local address) to another (the inside global address) 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. If you do not define any servers, NAT
offers the additional benefit of firewall protection. With no servers defined, your Device 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).
10.4.3 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
Device keeps track of the original addresses and port numbers so incoming reply packets can have
their original values restored. The following figure illustrates this.
Table 33 NAT Definitions
ITEM DESCRIPTION
Inside This refers to the host on the LAN.
Outside This refers to the host on the WAN.
Local This refers to the packet address (source or destination) as the packet travels on the
LAN.
Global This refers to the packet address (source or destination) as the packet travels on the
WAN.