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Customizing Your Network Settings

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7.

Type the gateway IP address, which must be a router on the same LAN segment as the

WNR3500v2 router.

8.

Type a number between 1 and 15 as the metric value.

This represents the number of routers between your network and the destination. Usually, a

setting of 2 or 3 works, but if this is a direct connection, set it to 1.

9.

Click

Apply

to have the static route entered into the table.

Allowing Inbound Connections to Your Network

By default, the WNR3500v2 router blocks any inbound traffic from the Internet to your computers

except for replies to your outbound traffic. However, you might need to create exceptions to this

rule for the following purposes:

•

To allow remote computers on the Internet to access a server on your local network.

•

To allow certain applications and games to work correctly when their replies are not

recognized by your router.

Your router provides two features for creating these exceptions: port forwarding and port

triggering. This section explains how a normal outbound connection works, followed by two

examples explaining how port forwarding and port triggering operate and how they differ.

How Your Computer Accesses a Remote Computer through Your

Router

When a computer on your network needs to access a computer on the Internet, your computer

sends your router a message containing source and destination address and process information.

Before forwarding your message to the remote computer, your router must modify the source

information and must create and track the communication session so that replies can be routed

back to your computer.

Here is an example of normal outbound traffic and the resulting inbound responses:

1.

You open Internet Explorer, beginning a browser session on your computer. Invisible to you,

your operating system assigns a service number (port number) to every communication

process running on your computer. In this example, let’s say Windows assigns port number

5678 to this browser session.

2.

You ask your browser to get a Web page from the Web server at www.example.com

.

Your

computer composes a Web page request message with the following address and

port information:

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•

The source address is your computer’s IP address.

•

The source port number is 5678, the browser session.

•

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

finds by asking a DNS server.

•

The destination port number is 80, the standard port number for a Web 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 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 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.

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•

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 senses the termination of 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 will not

recognize it and will discard 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,

“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 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.

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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 senses the termination of 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.

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. 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 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 you have defined:

Note:

An inbound port can be used by one computer at a time with the triggered

application.

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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

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

groups or 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 at a

time can use the same ports.

•

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.