Netgear DGN1000v3 User Manual download pdf (Page 101)

Advanced Settings

101

N150 Wireless ADSL2+ Modem Router DGN1000v3

Port Forwarding and Port Triggering

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

except replies to your outbound traffic. You might need to create exceptions to this rule for

these purposes:

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

• To enable certain applications and games to work correctly if the modem router does not

recognize their replies.

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

port triggering. The next sections provide background information to help you understand

how port forwarding and port triggering work, and the differences between the two.

Remote Computer Access Basics

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

computer sends your modem router a message containing the source and destination

address and process information. Before forwarding your message to the remote computer,

your modem router has to modify the source information and 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 a browser, and your operating system assigns port number 5678 to this

browser session.

2. You type http://www.example.com into the URL field, and your computer creates a web page

request message with the following address and port information. The request message is

sent to your modem router:

• Source address.

our computer’s IP address

• Source port number. 5678, which is the browser session

• Destination address.

The IP address of www

.example.com, which your computer

finds by asking a DNS server

• Destination port number. 80, which is the standard port number for a web server

process

3. Your modem 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 modem 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 modem router’

s public IP address.

This

requirement 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 modem router, such

as 33333. This requirement is necessary because two computers might

independently be using the same session number

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