Figure 15-1 How Nat Works - ZyXEL Communications ZyWALL 5 User Manual

ZyWALL 5 Internet Security Appliance
network and make them accessible to the outside world. Although you can make designated servers on
the LAN accessible to the outside world, it is strongly recommended that you attach those servers to
the DMZ port instead. 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).
15.1.3 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 15-1 How NAT Works

15.1.4 NAT Application
The following figure illustrates a possible NAT application, where three inside LANs (logical LANs
using IP Alias) behind the ZyWALL can communicate with three distinct WAN networks. More
examples follow at the end of this chapter.
15-2 NAT