How Nat Works; Figure 6-1 How Nat Works - ZyXEL Communications ZyXEL ZYWALL10 User Manual

ZyWALL 10 Internet Security Gateway
The global IP addresses for the inside hosts can be either static or dynamically assigned by the ISP. In
addition, you can designate servers, e.g., 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 (for Many-to-One and Many-to-Many
Overload mapping – see below), NAT offers the additional benefit of firewall protection. If no server is
defined in these cases, all incoming inquiries will be filtered out by your ZyWALL, 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 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 6-1 How NAT Works

6-2 NAT