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4.

The Web server at www.example.com composes a return message with the requested Web

page data. The return message contains the following address and port information:

•

The source address is the IP address of www.example.com.

•

The source port number is 80, the standard port number for a Web server process.

•

The destination address is the public IP address of your router.

•

The destination port number is 33333.

The Web server then sends this reply message to your router.

5.

Upon receiving the incoming message, your router checks its session table to determine

whether there is an active session for port number 33333. Finding an active session, the router

then modifies the message, restoring the original address information replaced by NAT. The

message now contains the following address and port information:

•

The source address is the IP address of www.example.com.

•

The source port number is 80, the standard port number for a Web server process.

•

The destination address is your computer’s IP address.

•

The destination port number is 5678, the browser session that made the initial request.

Your router then sends this reply message to your computer, which displays the Web page

from www.example.com.

6.

When you finish your browser session, your router eventually senses a period of inactivity in

the communications. Your router then removes the session information from its session table,

and incoming traffic is no longer accepted on port number 33333.

How Port Triggering Changes the Communication Process

In the preceding example, requests are sent to a remote computer by your router from a particular

service port number, and replies from the remote computer to your router are directed to that port

number. If the remote server sends a reply back to a different port number, your router does not

recognize it and discards it. However, some application servers (such as FTP and IRC servers)

send replies back to multiple port numbers. Using the port triggering function of your router, you

can tell the router to open additional incoming ports when a particular outgoing port originates a

session.

An example is Internet Relay Chat (IRC). Your computer connects to an IRC server at destination

port 6667. The IRC server not only responds to your originating source port, but also sends an

“identify” message to your computer on port 113. Using port triggering, you can tell the router,

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“When you initiate a session with destination port 6667, you must also allow incoming traffic on

port 113 to reach the originating computer.” Using steps similar to the preceding example, the

following sequence shows the effects of the port triggering rule that you have defined:

1.

You open an IRC client program, beginning 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 (as

in the previous example, let’s say 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 to determine whether there is 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 learns that there is 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 determing this information by contacting the publisher of the application or

user groups or newsgroups.

Note:

Only one computer at a time can use the triggered application.

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How Port Forwarding Changes the Communication Process

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. Usually, 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 our 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 that you have defined:

1.

The user of a remote computer opens Internet Explorer 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:

•

The destination address is the IP address of www.example.com, which is the address of

your router.

•

The destination port number is 80, 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 determine this information by contacting the publisher of the application or user

groups or newsgroups.

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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 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 must 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.

Configuring 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 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. The DMZ

server is configured in the WAN Setup screen, as discussed in

“Setting Up a Default DMZ Server”

on page 4-7

.

Before starting, you need to determine which type of service, application, or game you will

provide, and the local IP address of the computer that will provide the service. Be sure that the

computer’s IP address never changes.

Tip:

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

reserved IP address feature of your WNDR3300 router. See

“Using Address

Reservation” on page 4-4

for instructions on how to use reserved IP addresses.

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To configure port forwarding to a local server:

1.

Select

Port Forwarding/Port Triggering

under Advanced in the main menu. The

Forwarding/Port Triggering screen displays.

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 the following section,

“Adding a Custom

Service

.”

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.

Adding a Custom Service

To define a service, game, or application that does not appear in the Service Name list, you must

first determine which port number or range of numbers is used by the application. You can usually

determine this information by contacting the publisher of the application or user groups or

newsgroups. When you have the port number information, follow these steps:

1.

Select

Port Forwarding/Port Triggering

under Advanced in the main menu. The

Forwarding/Port Triggering screen displays.

Figure 5-1