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Note:

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

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 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.

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To configure port forwarding, you need to know which inbound ports the application needs.

You usually can determine this information by contacting the publisher of the application or

the relevant user groups and newsgroups.

How Port Forwarding Differs from Port Triggering

The following points summarize the differences between port forwarding and port triggering:

•

Port triggering can be used by any computer on your network, although only one

computer can use it at a time.

•

Port forwarding is configured for a single computer on your network.

•

Port triggering does require that you 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.

•

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

Using the port forwarding feature, you can 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 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 starting, determine which type of service, application, or game you want to provide.

Find out the local IP address of the computer that will provide the service. The server

computer has to always have the same IP address.



To set up port forwarding:

Tip:

To ensure that your server computer always has the same IP address,

use the reserved IP address feature of your N150R router.

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N150 WiFi Router (N150R)

1.

Select

Advanced > Port Forwarding/Port Triggering

to display the following screen:

Port Forwarding is selected as the service type.

2.

From the Service Name list, select the service or game that you will host on your network. If

the service does not appear in the list, see

Add a Custom Service

on page 48.

3.

In the corresponding Server IP Address field, enter the last digit of the IP address of your

local computer that will provide this service.

4.

Click

Add

. The service appears in the list in the screen.

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 get

this information by contacting the publisher of the application or user groups or newsgroups.



To add a custom service:

1.

Select

Advanced > Port Forwarding/Port Triggering

.

2.

Select

Port Forwarding

as the service type.

3.

Click the

Add Custom Service

button to display the following screen:

4.

In the Service Name field, enter a descriptive name.

5.

In the Protocol list, select the protocol. If you are unsure, select

TCP/UDP

.

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6.

In the Starting Port field, enter the beginning port number.

•

If the application uses a single port, enter the same port number in the Ending Port

field.

•

If the application uses a range of ports, enter the ending port number of the range in

the Ending Port field.

7.

In the Server IP Address field, enter the IP address of your local computer that will provide

this service.

8.

Click

Apply

. The service appears in the list in the Port Forwarding/Port Triggering screen.

Editing or Deleting a Port Forwarding Entry



To edit or delete a port forwarding entry:

1.

In the table, select the radio button next to the service name.

2.

Click

Edit Service

or

Delete Service

.

Application Example: Making 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 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 as described in

Dynamic DNS

on page 68. 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 myonnetworks.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

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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.

Note:

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) according to the instructions

in

Universal Plug and Play

on page 71.

To set up 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 can

usually get this information by contacting the publisher of the application or user groups or

newsgroups.



To set up port triggering:

1.

Select

Advanced > Port Forwarding/Port Triggering

.

2.

Select the

Port Triggering

radio button to display the port triggering information.

3.

Clear the

Disable Port Triggering

check box if it is selected.

Note:

If the Disable Port Triggering check box is selected after you configure

port triggering, port triggering is disabled. However, any port triggering

configuration information you added to the router is retained even though it is

not used.

4.

In the Port Triggering Timeout field, enter a value up to 9999 minutes.