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NETGEAR Wireless-N 300 Router WNR2000v2 User Manual
Fine-Tuning Your Network
5-2
v1.0, January 2010
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.
•
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.
Page 82 / 144
NETGEAR Wireless-N 300 Router WNR2000v2 User Manual
Fine-Tuning Your Network
5-3
v1.0, January 2010
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 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,
Page 83 / 144
NETGEAR Wireless-N 300 Router WNR2000v2 User Manual
Fine-Tuning Your Network
5-4
v1.0, January 2010
“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.
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.
Page 84 / 144
NETGEAR Wireless-N 300 Router WNR2000v2 User Manual
Fine-Tuning Your Network
5-5
v1.0, January 2010
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:
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.
Page 85 / 144
NETGEAR Wireless-N 300 Router WNR2000v2 User Manual
Fine-Tuning Your Network
5-6
v1.0, January 2010
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 4-8
.
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 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 WNR2000v2 router. See
“Using Address
Reservation” on page 4-5
for instructions on how to use reserved IP addresses.

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