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The Block Services Setup screen displays:

5.

From the Service Type list, select the application or service to allow or block.

The list displays several common services, but you are not limited to these choices. To

add any additional services or applications that do not already appear, select

User

Defined

.

6.

If you know that the application uses either TCP or UDP, select the appropriate protocol. If

you are not sure, select

TCP/UDB

(both).

7.

Enter the starting and ending port numbers.

If the application uses a single port number, enter that number in both fields.

8.

Select the radio button for the IP address configuration you want to block, and enter the IP

addresses.

You can block the specified service for a single computer, a range of computers with

consecutive IP addresses, or all computers on your network.

9.

Click the

Add

button.

Your changes are saved.

Port Triggering to Open Incoming Ports

Some application servers (such as FTP and IRC servers) send replies to multiple port

numbers. Using the port triggering function of your router, you can tell the router to open

more 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.

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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

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

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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 or news groups.

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.

•

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.

•

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.

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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

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.

If the service does not appear in the list, see

Add a Custom Service

on page

64.

4.

Fill in the Server IP Address field.

5.

Click the

Add

button.

The service appears in the list on the Port Forwarding 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

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

.

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

TCP/UDP

.

6.

In the External port range field, specify the port or port range.

7.

Either select the

Use the same port range for internal port

check box or fill in the Internal

IP address field.

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

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.