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3.

Your router creates an entry in its internal session table describing this communication

session between your computer and the web server at www.example.com. Before sending

the web page request message to www.example.com, your router stores the original

information and then modifies the source information in the request message, performing

Network Address Translation (NAT):

•

The source address is replaced with your router’s public IP address. This is

necessary because your computer uses a private IP address that is not globally

unique and cannot be used on the Internet.

•

The source port number is changed to a number chosen by the router, such as 33333.

This is necessary because two computers could independently be using the same

session number.

Your router then sends this request message through the Internet to the web server at

www.example.com.

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

web server then sends this reply message to your router.

Source address

. The IP address of www.example.com.

Source port number

. 80, which is the standard port number for a web server process.

Destination address

. The public IP address of your router.

Destination port number

. 33333.

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 to restore the original address information replaced by

NAT. Your router sends this reply message to your computer, which displays the web

page from www.example.com. The message now contains the following address and port

information.

Source address

. The IP address of www.example.com.

Source port number

. 80, which is the standard port number for a web server process.

Destination address

. Your computer’s IP address.

Destination port number

. 5678, which is the browser session that made the initial

request.

6.

When you finish your browser session, your router eventually detects 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.

Port Triggering to Open Incoming Ports

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

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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, “When you initiate a session with destination port 6667, you have to 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 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

(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 determine 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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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 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 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:

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•

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 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.

Configure 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, you need to 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.

Tip:

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

use the reserved IP address feature of your product.



To configure port forwarding:

1.

Select

Content Filtering > Port Forwarding/Port Triggering

to display the following

screen:

Figure 22.

Setting up port forwarding

2.

Select the

Port Forwarding

radio button as the service type.

3.

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 55.

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4.

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

local computer that will provide this service.

5.

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, you

have to 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

Content Filtering > Port Forwarding/Port Triggering

.

2.

Select the

Port Forwarding

radio button as the service type.

3.

Click the

Add Custom Service

button to display the following screen:

Figure 23.

Set up custom services

4.

In the

Service Name

field, enter a descriptive name.

5.

In the

Protocol

field, select the protocol. If you are unsure, select

TCP/UDP

.

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.

Edit or Delete a Port Forwarding Entry



To edit or delete a port forwarding entry:

1.

In the table, select the 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.

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