General Nat Screen - ZyXEL Communications NBG-418N User Manual

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 NBG-418N 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).
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 in each packet and then forwards it to the Internet. The
NBG-418N 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 73 How NAT Works

10.3 General NAT Screen

Use this screen to enable NAT and set a default server. Click Network > NAT to open the General
screen.
Figure 74 Network > NAT > General
NBG-418N User's Guide
Chapter 10 Network Address Translation
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