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Chapter 11 Network Address Translation (NAT)
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161
Figure 76
Multiple Servers Behind NAT Example
Click
Network Setting > NAT > Port Forwarding
to open the following screen.
See
Appendix D on page 309
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 55
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.
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.
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
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Chapter 11 Network Address Translation (NAT)
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162
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.
Table 56
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.
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 VMG 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.
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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
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.
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 56
Port Forwarding: Add/Edit (continued)
LABEL
DESCRIPTION
Table 57
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.
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Figure 80
Applications: Add
The following table describes the labels in this screen.
11.4
The Port Triggering Screen
Some services use a dedicated range of ports on the client side and a dedicated range of ports on
the server side. With regular port forwarding you set a forwarding port in NAT to forward a service
(coming in from the server on the WAN) to the IP address of a computer on the client side (LAN).
The problem is that port forwarding only forwards a service to a single LAN IP address. In order to
use the same service on a different LAN computer, you have to manually replace the LAN
computer's IP address in the forwarding port with another LAN computer's IP address.
Trigger port forwarding solves this problem by allowing computers on the LAN to dynamically take
turns using the service. The VMG records the IP address of a LAN computer that sends traffic to the
WAN to request a service with a specific port number and protocol (a "trigger" port). When the
VMG's WAN port receives a response with a specific port number and protocol ("open" port), the
VMG forwards the traffic to the LAN IP address of the computer that sent the request. After that
computer’s connection for that service closes, another computer on the LAN can use the service in
the same manner. This way you do not need to configure a new IP address each time you want a
different LAN computer to use the application.
For example:
Table 58
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 VMG 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.
Page 165 / 327
Chapter 11 Network Address Translation (NAT)
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165
Figure 81
Trigger Port Forwarding Process: Example
1
Jane requests a file from the Real Audio server (port 7070).
2
Port 7070 is a “trigger” port and causes the VMG to record Jane’s computer IP address. The VMG
associates Jane's computer IP address with the "open" port range of 6970-7170.
3
The Real Audio server responds using a port number ranging between 6970-7170.
4
The VMG forwards the traffic to Jane’s computer IP address.
5
Only Jane can connect to the Real Audio server until the connection is closed or times out. The VMG
times out in three minutes with UDP (User Datagram Protocol) or two hours with TCP/IP (Transfer
Control Protocol/Internet Protocol).
Click
Network Setting > NAT > Port Triggering
to open the following screen. Use this screen to
view your VMG’s trigger port settings.
Figure 82
Network Setting > NAT > Port Triggering
The following table describes the labels in this screen.
Table 59
Network Setting > NAT > Port Triggering
LABEL
DESCRIPTION
Add New Rule
Click this to create a new rule.
#
This is the index number of the entry.
Status
This field displays whether the port triggering 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 field displays the name of the service used by this rule.
WAN Interface
This field shows the WAN interface through which the service is forwarded.
Trigger Start
Port
The trigger port is a port (or a range of ports) that causes (or triggers) the VMG to record
the IP address of the LAN computer that sent the traffic to a server on the WAN.
This is the first port number that identifies a service.
Trigger End
Port
This is the last port number that identifies a service.
Trigger Proto.
This is the trigger transport layer protocol.

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