Zyxel NBG-416N Router Manual PDF (Setup & Configuration Guide)

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NBG-416N User’s Guide

101

C

HAPTER

11

Network Address Translation

(NAT)

11.1

Overview

This chapter discusses how to configure NAT on the NBG-416N.

NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP

address of a host in a packet. For example, the source address of an outgoing

packet, used within one network is changed to a different IP address known within

another network.

Each packet has two addresses – a source address and a destination address. For

outgoing packets, 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 NBG-

416N 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 59

NAT Example

A: 192.168.1.33

B: 192.168.1.34

C: 192.168.1.35

IP address

192.168.1.1

WAN

LAN

assigned by ISP

FTP, Telnet, SNMP

Port 80

Ports 21 to 25

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Chapter 11 Network Address Translation (NAT)

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For more information on IP address translation, refer to

RFC 1631

,

The IP Network

Address Translator (NAT)

.

Note: You must create a firewall rule in addition to setting up NAT, to allow traffic from

the WAN to be forwarded through the NBG-416N.

11.2

What You Can Do

•

Use the

General

screen to enable NAT and set a default server (

Section 11.3 on

page 104

).

•

Use the

Application

screen to change your NBG-416N’s port forwarding

settings (

Section 11.4 on page 105

).

11.2.1

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 NBG-416N, 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.

Table 40

NAT Definitions

ITEM

DESCRIPTION

Inside

This refers to the host on the LAN.

Outside

This refers to the host on the WAN.

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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 NBG-416N 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 NBG-416N keeps track of the original addresses and port numbers

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.

Table 40

NAT Definitions (continued)

ITEM

DESCRIPTION

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Chapter 11 Network Address Translation (NAT)

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so incoming reply packets can have their original values restored. The following

figure illustrates this.

Figure 60

How NAT Works

11.3

General NAT Screen

Use this screen to enable NAT and set a default server. Click

Network > NAT

to

open the

General

screen.

Figure 61

Network > NAT > General

The following table describes the labels in this screen.

Table 41

Network > NAT > General

LABEL

DESCRIPTION

NAT Setup

Enable Network

Address

Translation

Network Address Translation (NAT) allows the translation of an Internet

protocol address used within one network (for example a private IP

address used in a local network) to a different IP address known within

another network (for example a public IP address used on the Internet).

Select the check box to enable NAT.

Default Server Setup

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Chapter 11 Network Address Translation (NAT)

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11.4

NAT Application Screen

Use the

Application

screen to forward incoming service requests to the server(s)

on your local network. You may enter a single port number or a range of port

numbers to be forwarded, and the local IP address of the desired server. The port

number identifies a service; for example, web service is on port 80 and FTP on

port 21. In some cases, such as for unknown services or where one server can

support more than one service (for example both FTP and web service), it might

be better to specify a range of port numbers.

In addition to the servers for specified services, NAT supports a default server. A

service request that does not have a server explicitly designated for it is forwarded

to the default server. If the default is not defined, the service request is simply

discarded.

Note: Many residential broadband ISP accounts do not allow you to run any server

processes (such as a Web or FTP server) from your location. Your ISP may

periodically check for servers and may suspend your account if it discovers any

active services at your location. If you are unsure, refer to your ISP.

Port forwarding allows you to define the local servers to which the incoming

services will be forwarded. To change your NBG-416N’s port forwarding settings,

click

Network > NAT

>

Application

. The screen appears as shown.

Note: If you do not assign a

Default Server

IP address

in the

NAT >

General

screen,

the NBG-416N discards all packets received for ports that are not specified in

this screen or remote management.

Server IP

Address

In addition to the servers for specified services, NAT supports a default

server. A default server receives packets from ports that are not

specified in the

Application

screen.

If you do not assign a

Default

Server

IP address

, the NBG-416N

discards all packets received for ports that are not specified in the

Application

screen or remote management.

Apply

Click

Apply

to save your changes back to the NBG-416N.

Reset

Click

Reset

to begin configuring this screen afresh.

Table 41

Network > NAT > General (continued)

LABEL

DESCRIPTION

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