ZyXEL Communications Parental Control Gateway HS100/HS100W - Manual (Page 132)

HomeSafe User’s Guide
9-2 NAT Screens
(for Many-to-One and Many-to-Many Overload mapping), NAT offers the additional benefit of
firewall protection. With no servers defined, your HomeSafe 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).
9.1.3 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 HomeSafe 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 9-1 How NAT Works
9.1.4 NAT Application
The following figure illustrates a possible NAT application, where three inside LANs (logical
LANs using IP Alias) behind the HomeSafe can communicate with three distinct WAN networks.
More examples follow at the end of this chapter.