Zyxel VMG1312-B10D Router Manual PDF (Setup & Configuration Guide)

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Chapter 11 Network Address Translation (NAT)

VMG1312-B Series User’s Guide

176

11.7.1

Add/Edit Address Mapping Rule

To add or edit an address mapping rule, click

Add new rule

or the rule’s edit icon in the

Address

Mapping

screen to display the screen shown next.

Figure 87

Address Mapping: Add/Edit

The following table describes the fields in this screen.

Type

This is the address mapping type.

One-to-One

: This mode maps one local IP address to one global IP address. Note that port

numbers do not change for the One-to-one NAT mapping type.

Many-to-One

: This mode maps multiple local IP addresses to one global IP address. This is

equivalent to SUA (i.e., PAT, port address translation), the Device's Single User Account

feature that previous routers supported only.

Many-to-Many

: This mode maps multiple local IP addresses to shared global IP addresses.

Modify

Click the

Edit

icon to go to the screen where you can edit the address mapping rule.

Click the

Delete

icon to delete an existing address mapping rule. Note that subsequent

address mapping rules move up by one when you take this action.

Table 62

Network Setting > NAT > Address Mapping (continued)

LABEL

DESCRIPTION

Table 63

Address Mapping: Add/Edit

LABEL

DESCRIPTION

Type

Choose the IP/port mapping type from one of the following.

One-to-One

: This mode maps one local IP address to one global IP address. Note that port

numbers do not change for the One-to-one NAT mapping type.

Many-to-One

: This mode maps multiple local IP addresses to one global IP address. This is

equivalent to SUA (i.e., PAT, port address translation), the Device's Single User Account

feature that previous routers supported only.

Many-to-Many

: This mode maps multiple local IP addresses to shared global IP addresses.

Local Start IP

Enter the starting Inside Local IP Address (ILA).

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Chapter 11 Network Address Translation (NAT)

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177

11.8

The Sessions Screen

Use this screen to limit the number of concurrent NAT sessions a client can use. Click

Network

Setting > NAT > Sessions

to display the following screen.

Figure 88

Network Setting > NAT > Sessions

The following table describes the fields in this screen.

11.9

Technical Reference

This part contains more information regarding NAT.

Local End IP

Enter the ending Inside Local IP Address (ILA). If the rule is for all local IP addresses, then

this field displays 0.0.0.0 as the Local Start IP address and 255.255.255.255 as the Local

End IP address. This field is blank for

One-to-One

mapping types.

Global Start IP

Enter the starting Inside Global IP Address (IGA). Enter 0.0.0.0 here if you have a dynamic

IP address from your ISP. You can only do this for the

Many-to-One

mapping type.

Global End IP

Enter the ending Inside Global IP Address (IGA). This field is blank for

One-to-One

and

Many-to-One

mapping types.

Set

Select the number of the mapping set for which you want to configure.

OK

Click

OK

to save your changes.

Cancel

Click

Cancel

to exit this screen without saving.

Table 63

Address Mapping: Add/Edit (continued)

LABEL

DESCRIPTION

Table 64

Network Setting > NAT > Sessions

LABEL

DESCRIPTION

WAX NAT

Session Per

Host

Use this field to set a limit to the number of concurrent NAT sessions each client host can

have.

If only a few clients use peer to peer applications, you can raise this number to improve

their performance. With heavy peer-to-peer application use, lower this number to ensure no

single client uses too many of the available NAT sessions.

Apply

Click this to save your changes on this screen.

Cancel

Click this to exit this screen without saving any changes.

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Chapter 11 Network Address Translation (NAT)

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11.9.1

NAT Definitions

Inside/outside denotes where a host is located relative to the Device, for example, the computers

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

hosts.

Global/local 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 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.

11.9.2

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 (for Many-

to-One and Many-to-Many Overload mapping), NAT offers the additional benefit of firewall

protection. With no servers defined, your 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)

.

Table 65

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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11.9.3

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

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.

Figure 89

How NAT Works

11.9.4

NAT Application

The following figure illustrates a possible NAT application, where three inside LANs (logical LANs

using IP alias) behind the Device can communicate with three distinct WAN networks.

192.168.1.13

192.168.1.10

192.168.1.11

192.168.1.12

SA

192.168.1.10

SA

IGA1

Inside Local

IP Address

192.168.1.10

192.168.1.11

192.168.1.12

192.168.1.13

Inside Global

IP Address

IGA 1

IGA 2

IGA 3

IGA 4

NAT Table

WAN

LAN

Inside Local

Address (ILA)

Inside Global

Address (IGA)

Page 180 / 331

Chapter 11 Network Address Translation (NAT)

VMG1312-B Series User’s Guide

180

Figure 90

NAT Application With IP Alias

Port Forwarding: Services and Port Numbers

The most often used port numbers are shown in the following table. Please refer to RFC 1700 for

further information about port numbers. Please also refer to the Supporting CD for more examples

and details on port forwarding and NAT.

Port Forwarding Example

Let's say you want to assign ports 21-25 to one FTP, Telnet and SMTP server (

A

in the example),

port 80 to another (

B

in the example) and assign a default server IP address of 192.168.1.35 to a

Table 66

Services and Port Numbers

SERVICES

PORT NUMBER

ECHO

7

FTP (File Transfer Protocol)

21

SMTP (Simple Mail Transfer Protocol)

25

DNS (Domain Name System)

53

Finger

79

HTTP (Hyper Text Transfer protocol or WWW, Web)

80

POP3 (Post Office Protocol)

110

NNTP (Network News Transport Protocol)

119

SNMP (Simple Network Management Protocol)

161

SNMP trap

162

PPTP (Point-to-Point Tunneling Protocol)

1723

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