N600 Wireless Dual Band Router WNDR3400 User Manual

Customizing Your Network Settings

5-9

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3.

Click

Apply

.

Configuring Static Routes

Static routes provide additional routing information to your router. Under usual circumstances, the

router has adequate routing information after it has been configured for Internet access, and you do

not need to configure additional static routes. You must configure static routes only for unusual

cases such as multiple routers or multiple IP subnets located on your network.

As an example of when a static route is needed, consider the following case:

•

Your primary Internet access is through a cable modem to an ISP.

•

You have an ISDN router on your home network for connecting to the company where you are

employed. This router’s address on your LAN is

1

92.168.1.100.

•

Your company’s network address is 134.177.0.0.

When you first configured your router, two implicit static routes were created. A default route was

created with your ISP as the gateway, and a second static route was created to your local network

for all 192.168.1.x addresses. With this configuration, if you attempt to access a device on the

134.177.0.0 network, your router forwards your request to the ISP. The ISP forwards your request

to the company where you are employed, and the request is likely to be denied by the company’s

firewall.

In this case you must define a static route, telling your router that 134.177.0.0 should be accessed

through the ISDN router at 192.168.1.100.

In this example:

•

The

Destination IP Address

and

IP Subnet Mask

fields specify that this static route applies

to all 134.177.x.x addresses.

•

The

Gateway IP Address

field specifies that all traffic for these addresses should be

forwarded to the ISDN router at 192.168.1.100.

•

A

Metric

value of 1 will work since the ISDN router is on the LAN.

•

Private

is selected only as a precautionary security measure in case RIP is activated.

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Select

Static Routes

under Advanced in the main menu. The Static Routes screen displays.

To add or edit a static route:

1.

Click

Add

to open the Static Routes screen.

2.

In the

Route Name

field, type a name for this static route. (This is for identification purposes

only.)

3.

Select the

Private

check box if you want to limit access to the LAN only. If Private is selected,

the static route is not reported in RIP.

4.

Select the

Active

check box to make this route effective.

5.

Type the destination IP address of the final destination.

6.

Type the IP subnet mask for this destination.

If the destination is a single host, type 255.255.255.255.

7.

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

WNDR3400 router.

Figure 5-4

Figure 5-5

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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 WNDR3400 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:

•

The source address is your computer’s IP address.

•

The source port number is 5678, the browser session.

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•

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.

•

The destination address is your computer’s IP address.

•

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

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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 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.

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.

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