Figure 8-1 How Nat Works - ZyXEL Communications ZYWALL2 ET 2WE User Manual

ZyWALL 2 Series User's Guide
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. You can make designated servers on the LAN accessible to the
outside world. If you do not define any servers (for Many-to-One and Many-to-Many Overload mapping),
NAT offers the additional benefit of firewall protection. With no servers defined, your ZyWALL 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).
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 ZyWALL keeps track of the original addresses and port numbers so incoming
reply packets can have their original values restored. The following figure illustrates this.

Figure 8-1 How NAT Works

8-2 NAT