ZyXEL Communications P-660HN-F1 User Manual

Chapter 8 Network Address Translation (NAT)

P-660HN-F1 User’s Guide

Note that inside/outside refers to the location of a host, while global/local refers to the IP 
address of a host used in a packet. Thus, an inside local address (ILA) is the IP address of an 
inside host in a packet when the packet is still in the local network, while an inside global 
address (IGA) is the IP address of the same inside host when the packet is on the WAN side. 
The following table summarizes this information.

NAT never changes the IP address (either local or global) of an outside host.

In the simplest form, NAT changes the source IP address in a packet received from a 
subscriber (the inside local address) to another (the inside global address) before forwarding 
the packet to the WAN side. When the response comes back, NAT translates the destination 
address (the inside global address) back to the inside local address before forwarding it to the 
original inside host. Note that the IP address (either local or global) of an outside host is never 
changed.

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 (for Many-to-One and Many-to-Many Overload mapping – see 

NAT offers the additional benefit of firewall protection. With no servers defined, your ZyXEL 
Device 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).

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 ZyXEL Device keeps track of the original addresses 
and port numbers so incoming reply packets can have their original values restored. The 
following figure illustrates this.

Table 50   NAT Definitions

Inside

This refers to the host on the LAN.

Outside

This refers to the host on the WAN.

Local

This refers to the packet address (source or destination) as the packet travels on the 
LAN.

Global

This refers to the packet address (source or destination) as the packet travels on the 
WAN.