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Page 166 / 331 Scroll up to view Page 161 - 165
Chapter 11 Network Address Translation (NAT)
VMG1312-B Series User’s Guide
166
NAT
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.
Port Forwarding
A port forwarding set is a list of inside (behind NAT on the LAN) servers, for example, web or FTP,
that you can make visible to the outside world even though NAT makes your whole inside network
appear as a single computer to the outside world.
Finding Out More
See
Section 11.9 on page 177
for advanced technical information on NAT.
11.2
The Port Forwarding Screen
Use the
Port Forwarding
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. You can allocate a server IP address that corresponds to a port or
a range of ports.
The most often used port numbers and services are shown in
Appendix D on page 314
. Please refer
to RFC 1700 for further information about port numbers.
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.
Configuring Servers Behind 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
third (
C
in the example). You assign the LAN IP addresses and the ISP assigns the WAN IP address.
The NAT network appears as a single host on the Internet.
Page 167 / 331
Chapter 11 Network Address Translation (NAT)
VMG1312-B Series User’s Guide
167
Figure 76
Multiple Servers Behind NAT Example
Click
Network Setting > NAT > Port Forwarding
to open the following screen.
See
Appendix D on page 314
for port numbers commonly used for particular services.
Figure 77
Network Setting > NAT > Port Forwarding
The following table describes the fields in this screen.
Table 54
Network Setting > NAT > Port Forwarding
LABEL
DESCRIPTION
Add new rule
Click this to add a new rule.
#
This is the index number of the entry.
Status
This field displays whether the NAT rule is active or not. A yellow bulb signifies that this rule
is active. A gray bulb signifies that this rule is not active.
Service Name
This shows the service’s name.
WAN Interface
This shows the WAN interface through which the service is forwarded.
WAN IP
This field displays the incoming packet’s destination IP address.
Server IP
Address
This is the server’s IP address.
Start Port
This is the first external port number that identifies a service.
A=192.168.1.33
D=192.168.1.36
C=192.168.1.3
B=192.168.1.34
WAN
LAN
192.168.1.1
IP Address assigned by ISP
Page 168 / 331
Chapter 11 Network Address Translation (NAT)
VMG1312-B Series User’s Guide
168
11.2.1
Add/Edit Port Forwarding
Click
Add new rule
in the
Port Forwarding
screen or click the
Edit
icon next to an existing rule to
open the following screen.
Figure 78
Port Forwarding: Add/Edit
The following table describes the labels in this screen.
End Port
This is the last external port number that identifies a service.
Translation
Start Port
This is the first internal port number that identifies a service.
Translation End
Port
This is the last internal port number that identifies a service.
Protocol
This shows the IP protocol supported by this virtual server, whether it is
TCP
,
UDP
, or
TCP/
UDP
.
Modify
Click the
Edit
icon to edit this rule.
Click the
Delete
icon to delete an existing rule.
Table 54
Network Setting > NAT > Port Forwarding (continued)
LABEL
DESCRIPTION
Table 55
Port Forwarding: Add/Edit
LABEL
DESCRIPTION
Active
Clear the checkbox to disable the rule. Select the check box to enable it.
Service Name
Enter a name to identify this rule using keyboard characters (A-Z, a-z, 1-2 and so on).
WAN Interface
Select the WAN interface through which the service is forwarded.
You must have already configured a WAN connection with NAT enabled.
Page 169 / 331
Chapter 11 Network Address Translation (NAT)
VMG1312-B Series User’s Guide
169
11.3
The Applications Screen
This screen provides a summary of all NAT applications and their configuration. In addition, this
screen allows you to create new applications and/or remove existing ones.
To access this screen, click
Network Setting > NAT > Applications
. The following screen
appears.
Figure 79
Network Setting > NAT > Applications
WAN IP
Enter the WAN IP address for which the incoming service is destined. If the packet’s
destination IP address doesn’t match the one specified here, the port forwarding rule will
not be applied.
Start Port
Enter the original destination port for the packets.
To forward only one port, enter the port number again in the
End Port
field.
To forward a series of ports, enter the start port number here and the end port number in
the
End Port
field.
End Port
Enter the last port of the original destination port range.
To forward only one port, enter the port number in the
Start
Port
field above and then
enter it again in this field.
To forward a series of ports, enter the last port number in a series that begins with the port
number in the
Start Port
field above.
Translation
Start Port
This shows the port number to which you want the Device to translate the incoming port.
For a range of ports, enter the first number of the range to which you want the incoming
ports translated.
Translation End
Port
This shows the last port of the translated port range.
Server IP
Address
Enter the inside IP address of the virtual server here.
Protocol
Select the protocol supported by this virtual server. Choices are
TCP
,
UDP
, or
TCP/UDP
.
OK
Click
OK
to save your changes.
Cancel
Click
Cancel
to exit this screen without saving.
Table 55
Port Forwarding: Add/Edit (continued)
LABEL
DESCRIPTION
Page 170 / 331
Chapter 11 Network Address Translation (NAT)
VMG1312-B Series User’s Guide
170
The following table describes the labels in this screen.
11.3.1
Add New Application
This screen lets you create new NAT application rules. Click
Add new application
in the
Applications
screen to open the following screen.
Figure 80
Applications: Add
The following table describes the labels in this screen.
Table 56
Network Setting > NAT > Applications
LABEL
DESCRIPTION
Add new
application
Click this to add a new NAT application rule.
Application
Forwarded
This field shows the type of application that the service forwards.
WAN Interface
This field shows the WAN interface through which the service is forwarded.
Server IP
Address
This field displays the destination IP address for the service.
Modify
Click the
Delete
icon to delete the rule.
Table 57
Applications: Add
LABEL
DESCRIPTION
WAN Interface
Select the WAN interface that you want to apply this NAT rule to.
Server IP
Address
Enter the inside IP address of the application here.
Application
Category
Select the category of the application from the drop-down list box.
Application
Forwarded
Select a service from the drop-down list box and the Device automatically configures the
protocol, start, end, and map port number that define the service.
View Rule
Click this to display the configuration of the service that you have chosen in
Application
Fowarded
.
OK
Click
OK
to save your changes.
Cancel
Click
Cancel
to exit this screen without saving.

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