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Chapter 11 Network Address Translation (NAT)
VMG1312-B Series User’s Guide
171
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 Device 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
Device's WAN port receives a response with a specific port number and protocol ("open" port), the
Device 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:
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 Device to record Jane’s computer IP address. The
Device 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 Device 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
Device 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 Device’s trigger port settings.
Page 172 / 331
Chapter 11 Network Address Translation (NAT)
VMG1312-B Series User’s Guide
172
Figure 82
Network Setting > NAT > Port Triggering
The following table describes the labels in this screen.
11.4.1
Add/Edit Port Triggering Rule
This screen lets you create new port triggering rules. Click
Add new rule
in the
Port Triggering
screen or click a rule’s
Edit
icon to open the following screen.
Table 58
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 Device 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.
Open Start Port
The open port is a port (or a range of ports) that a server on the WAN uses when it sends
out a particular service. The Device forwards the traffic with this port (or range of ports) to
the client computer on the LAN that requested the service.
This is the first port number that identifies a service.
Open End Port
This is the last port number that identifies a service.
Open Proto.
This is the open transport layer protocol.
Modify
Click the
Edit
icon to edit this rule.
Click the
Delete
icon to delete an existing rule.
Page 173 / 331
Chapter 11 Network Address Translation (NAT)
VMG1312-B Series User’s Guide
173
Figure 83
Port Triggering: Add/Edit
The following table describes the labels in this screen.
11.5
The DMZ Screen
In addition to the servers for specified services, NAT supports a default server IP address. A default
server receives packets from ports that are not specified in the
NAT Port Forwarding Setup
screen.
Table 59
Port Triggering: Configuration Add/Edit
LABEL
DESCRIPTION
Active
Select the check box to enable this rule.
Service Name
Enter a name to identify this rule using keyboard characters (A-Z, a-z, 1-2 and so on).
WAN Interface
Select a WAN interface for which you want to configure port triggering rules.
Trigger Start
Port
The trigger port is a port (or a range of ports) that causes (or triggers) the Device to record
the IP address of the LAN computer that sent the traffic to a server on the WAN.
Type a port number or the starting port number in a range of port numbers.
Trigger End
Port
Type a port number or the ending port number in a range of port numbers.
Trigger Protocol
Select the transport layer protocol from
TCP
,
UDP
, or
TCP/UDP
.
Open Start Port
The open port is a port (or a range of ports) that a server on the WAN uses when it sends
out a particular service. The Device forwards the traffic with this port (or range of ports) to
the client computer on the LAN that requested the service.
Type a port number or the starting port number in a range of port numbers.
Open End Port
Type a port number or the ending port number in a range of port numbers.
Open Protocol
Select the transport layer protocol from
TCP
,
UDP
, or
TCP/UDP
.
OK
Click
OK
to save your changes.
Cancel
Click
Cancel
to exit this screen without saving.
Page 174 / 331
Chapter 11 Network Address Translation (NAT)
VMG1312-B Series User’s Guide
174
Figure 84
Network Setting > NAT > DMZ
The following table describes the fields in this screen.
11.6
The ALG Screen
Some NAT routers may include a SIP Application Layer Gateway (ALG). A SIP ALG allows SIP calls
to pass through NAT by examining and translating IP addresses embedded in the data stream.
When the Device registers with the SIP register server, the SIP ALG translates the Device’s private
IP address inside the SIP data stream to a public IP address. You do not need to use STUN or an
outbound proxy if your Device is behind a SIP ALG.
Use this screen to enable and disable the NAT and SIP (VoIP) ALG in the Device. To access this
screen, click
Network Setting > NAT > ALG
.
Figure 85
Network Setting > NAT > ALG
Table 60
Network Setting > NAT > DMZ
LABEL
DESCRIPTION
Default Server
Address
Enter the IP address of the default server which receives packets from ports that are not
specified in the
NAT Port Forwarding
screen.
Note: If you do not assign a
Default Server Address
, the Device discards all packets
received for ports that are not specified in the
NAT Port Forwarding
screen.
Apply
Click
Apply
to save your changes.
Cancel
Click
Cancel
to restore your previously saved settings.
Page 175 / 331
Chapter 11 Network Address Translation (NAT)
VMG1312-B Series User’s Guide
175
The following table describes the fields in this screen.
11.7
The Address Mapping Screen
Ordering your rules is important because the Device applies the rules in the order that you specify.
When a rule matches the current packet, the Device takes the corresponding action and the
remaining rules are ignored.
Click
Network Setting > NAT > Address Mapping
to display the following screen.
Figure 86
Network Setting > NAT > Address Mapping
The following table describes the fields in this screen.
Table 61
Network Setting > NAT > ALG
LABEL
DESCRIPTION
NAT ALG
Enable this to make sure applications such as FTP and file transfer in IM applications work
correctly with port-forwarding and address-mapping rules.
SIP ALG
Enable this to make sure SIP (VoIP) works correctly with port-forwarding and address-
mapping rules.
RTSP ALG
Enable this to have the Device detect RTSP traffic and help build RTSP sessions through its
NAT. The Real Time Streaming (media control) Protocol (RTSP) is a remote control for
multimedia on the Internet.
Apply
Click
Apply
to save your changes.
Cancel
Click
Cancel
to restore your previously saved settings.
Table 62
Network Setting > NAT > Address Mapping
LABEL
DESCRIPTION
Add new rule
Click this to create a new rule.
Set
This is the index number of the address mapping set.
Local Start IP
This is the starting Inside Local IP Address (ILA).
Local End IP
This is 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
This is 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
This is the ending Inside Global IP Address (IGA). This field is blank for
One-to-One
and
Many-to-One
mapping types.

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