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preceding example, the following sequence shows the effects of the port triggering rule you
have defined:
1.
You open an IRC client program to start a chat session on your computer.
2.
Your IRC client composes a request message to an IRC server using a destination port
number of 6667, the standard port number for an IRC server process. Your computer then
sends this request message to your router.
3.
Your router creates an entry in its internal session table describing this communication
session between your computer and the IRC server. Your router stores the original
information, performs Network Address Translation (NAT) on the source address and port,
and sends this request message through the Internet to the IRC server.
4.
Noting your port triggering rule and having observed the destination port number of 6667,
your router creates an additional session entry to send any incoming port 113 traffic to your
computer.
5.
The IRC server sends a return message to your router using the NAT-assigned source port
(for example, port 33333) as the destination port. The IRC server also sends an “identify”
message to your router with destination port 113.
6.
Upon receiving the incoming message to destination port 33333, your router checks its
session table for an active session for port number 33333. Finding an active session, the
router restores the original address information replaced by NAT and sends this reply
message to your computer.
7.
Upon receiving the incoming message to destination port 113 your router checks its session
table and finds an active session for port 113, associated with your computer. The router
replaces the message’s destination IP address with your computer’s IP address and
forwards the message to your computer.
8.
When you finish your chat session, your router eventually senses a period of inactivity in the
communications. The router then removes the session information from its session table,
and incoming traffic is no longer accepted on port numbers 33333 or 113.
To configure port triggering, you need to know which inbound ports the application needs.
Also, you need to know the number of the outbound port that will trigger the opening of the
inbound ports. You can usually determine this information by contacting the publisher of the
application or the relevant user groups or news groups.
Only one computer at a time can use the triggered application.
Port Forwarding to Permit External Host
Communications
In both of the preceding examples, your computer initiates an application session with a
server computer on the Internet. However, you might need to allow a client computer on the
Internet to initiate a connection to a server computer on your network. Normally, your router
ignores any inbound traffic that is not a response to your own outbound traffic. You can
configure exceptions to this default rule by using the port forwarding feature.
A typical application of port forwarding can be shown by reversing the client-server
relationship from the previous web server example. In this case, a remote computer’s
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browser needs to access a web server running on a computer in your local network. Using
port forwarding, you can tell the router, “When you receive incoming traffic on port 80 (the
standard port number for a web server process), forward it to the local computer at
192.168.1.123.” The following sequence shows the effects of the port forwarding rule you
have defined:
1.
The user of a remote computer opens a browser and requests a web page from
www.example.com, which resolves to the public IP address of your router. The remote
computer composes a web page request message with the following destination
information:
Destination address
. The IP address of www.example.com, which is the address of
your router.
Destination port number
. 80, which is the standard port number for a web server
process.
The remote computer then sends this request message through the Internet to your
router.
2.
Your router receives the request message and looks in its rules table for any rules covering
the disposition of incoming port 80 traffic. Your port forwarding rule specifies that incoming
port 80 traffic should be forwarded to local IP address 192.168.1.123. Therefore, your router
modifies the destination information in the request message:
The destination address is replaced with 192.168.1.123.
Your router then sends this request message to your local network.
3.
Your web server at 192.168.1.123 receives the request and composes a return message
with the requested web page data. Your web server then sends this reply message to your
router.
4.
Your router performs Network Address Translation (NAT) on the source IP address, and
sends this request message through the Internet to the remote computer, which displays the
web page from www.example.com.
To configure port forwarding, you need to know which inbound ports the application needs.
You can usually find this information by contacting the publisher of the application or the
relevant user groups or news groups.
How Port Forwarding Differs from Port Triggering
The following points summarize the differences between port forwarding and port triggering:
Any computer on your network can use port triggering, although only one computer can
use it at a time.
Port forwarding is configured for a single computer on your network.
With port triggering, the router does not need to know the computer’s IP address in
advance. The IP address is captured automatically.
Port forwarding requires that you specify the computer’s IP address during configuration,
and the IP address can never change.
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Port triggering requires specific outbound traffic to open the inbound ports, and the
triggered ports are closed after a period of no activity.
Port forwarding is always active and does not need to be triggered.
Set Up Port Forwarding to Local Servers
The port forwarding feature lets you allow certain types of incoming traffic to reach servers on
your local network. For example, you might want to make a local web server, FTP server, or
game server visible and available to the Internet.
Use the Port Forwarding/Port Triggering screen to configure the router to forward specific
incoming protocols to computers on your local network. In addition to servers for specific
applications, you can also specify a default DMZ server to which all other incoming protocols
are forwarded.
Before you start, determine which type of service, application, or game you want to provide,
and the local IP address of the computer that will provide the service. The server computer
has to always have the same IP address.
To ensure that your server computer always has the same IP address, use the reserved IP
address feature of your product. See
Set Up Address Reservation
on page
38.
To forward specific incoming protocols:
1.
Select
ADVANCED > Advanced Setup > Port Forwarding/Port Triggering
.
2.
Leave the
Port Forwarding
radio button selected as the service type.
3.
In the Service Name list, select the service name.
4.
Click the
Add
button.
A list of services displays. If the service that you want to add is not in the list, see
Add a
Custom Service
on page
74.
5.
Fill in the Server IP Address field.
6.
Click the
Add
button.
The service appears in the list on the Port Forwarding/Port Triggering screen.
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Add a Custom Service
To define a service, game, or application that does not appear in the Service Name list, first
determine which port number or range of numbers the application uses. You can usually
determine this information by contacting the publisher of the application or user groups or
news groups. When you have the port number information, follow these steps.
To add a custom service:
1.
Select
ADVANCED > Advanced Setup > Port Forwarding/Port Triggering
.
2.
Select the
Port Forwarding
radio button as the service type.
3.
Click the
Add Custom Service
button.
4.
In the Service Name field, enter a descriptive name.
5.
In the Service Type field, select the protocol. If you are unsure, select
TCP/UDP
.
6.
In the External Starting Port field, enter the beginning port number.
If the application uses a single port, enter the same port number in the External
Ending Port field.
If the application uses a range of ports, enter the ending port number of the range in
the External Ending Port field.
7.
Specify the internal ports by one of these methods:
Leave the
Use the same port range for Internal port
check box selected.
Fill in the Internal Starting Port and Internal Ending Port fields.
8.
Click the
Apply
button.
The service appears in the list in the Port Forwarding/Port Triggering screen.
Edit or Delete a Port Forwarding Entry
To edit or delete a port forwarding entry:
1.
Select
ADVANCED > Advanced Setup > Port Forwarding/Port Triggering
.
2.
Select the
Port Forwarding
radio button as the service type
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3.
In the table, select the radio button next to the service name.
4.
Click the
Edit Service
or
Delete Service
button.
Application Example: Make a Local Web Server Public
If you host a web server on your local network, you can use port forwarding to allow web
requests from anyone on the Internet to reach your web server.
To make a local web server public:
1.
Assign your web server either a fixed IP address or a dynamic IP address using DHCP
address reservation.
In this example, your router always gives your web server an IP address of 192.168.1.33.
2.
In the Port Forwarding/Port Triggering screen, configure the router to forward the HTTP
service to the local address of your web server at
192.168.1.33
.
HTTP (port 80) is the standard protocol for web servers.
3.
(Optional) Register a host name with a Dynamic DNS service, and configure your router to
use the name.
To access your web server from the Internet, a remote user has to know the IP address
that your ISP assigned. However, if you use a Dynamic DNS service, the remote user can
reach your server by a user-friendly Internet name, such as mynetgear.dyndns.org.
Set Up Port Triggering
Port triggering is a dynamic extension of port forwarding that is useful in these cases:
More than one local computer needs port forwarding for the same application (but not
simultaneously).
An application needs to open incoming ports that are different from the outgoing port.
When port triggering is enabled, the router monitors outbound traffic looking for a specified
outbound “trigger” port. When the router detects outbound traffic on that port, it remembers
the IP address of the local computer that sent the data. The router then temporarily opens the
specified incoming port or ports and forwards incoming traffic on the triggered ports to the
triggering computer.
Port forwarding creates a static mapping of a port number or range to a single local computer.
Port triggering can dynamically open ports to any computer that needs them and can close
the ports when they are no longer needed.
If you use applications such as multiplayer gaming, peer-to-peer connections, real-time
communications such as instant messaging, or remote assistance (a feature in Windows
XP), you should also enable Universal Plug and Play (UPnP). See
Universal Plug and Play
on page
104.
To configure port triggering, you need to know which inbound ports the application needs,
and the number of the outbound port that will trigger the opening of the inbound ports. You

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