General - ZyXEL Communications NBG5615 User Manual

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 NBG5615 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
NBG5615 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 82 How NAT Works

15.2 General

Use this screen to enable NAT and set a default server. Click Network > NAT to open the General
screen.
Figure 83 Network > NAT > General
NBG5615 User's Guide
Chapter 15 NAT
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