ZyXEL NBG4615 User Manual download pdf (Page 172)

Chapter 20 NAT

NBG4615 User’s Guide

172

•Use the Advanced screen to change your NBG4615’s trigger port settings (Section 20.5.3 on

page 178).

20.1.2 What You Need To Know

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

Inside/Outside

This denotes where a host is located relative to the NBG4615, for example, the computers of your

subscribers are the inside hosts, while the web servers on the Internet are the outside hosts.

Global/Local

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.

What NAT Does

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 NBG4615 filters out

Table 68 NAT Definitions

ITEM DESCRIPTION

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.

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ZyXEL NBG4615 User Manual Page 172