ZyXEL nbg-5715 Guia Do Utilizador

Chapter 14 NAT

NBG5715 User’s Guide

• Use  the  NAT Advance screen to change your NBG5715’s trigger port settings (

).

The following terms and concepts may help as you read through this chapter.

This denotes where a host is located relative to the NBG5715, for example, the computers of your 
subscribers are the inside hosts, while the web servers on the Internet are the outside hosts. 

This denotes the IP address of a host in a packet as the packet traverses a router, for example, the 
local address refers to the IP address of a host when the packet is in the local network, while the 
global address refers to the IP address of the host when the same packet is traveling in the WAN 
side. 

Note: Inside/outside refers to the location of a host, while global/local refers to the IP 

address of a host used in a packet. 

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.

Note: 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 , NAT 
offers the additional benefit of firewall protection. With no servers defined, your NBG5715 filters out 

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.