How Port Forwarding Changes The Communication Process - NETGEAR WNR1000v3h2 User Manual

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.

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.
60 | Chapter 5: Fine-Tuning Your Network
N150 Wireless Router User Manual