User's Manual

Configure Advanced Features
118
AC1900, N900, and N450 WiFi Cable Data Gateways
Here is an example of normal outbound traffic and the resulting inbound responses:
1. Y
ou open a browser, and your operating system assigns port number 5678 to this
browser session.
2. Y
ou type http://www.example.com in the URL field, and your computer creates a web page
request message that it sends to your cable data gateway. The message contains the
following information:
• Source address.
Your 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. Y
our cable data gateway 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 cable data
gateway 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 cable data gateway’
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 cable data gateway,
such as 33333.
This requirement is necessary because two computers might
independently be using the same port number.
Your cable data gateway 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 web server then sends this reply message to your cable data gateway.
The return message contains the following address and port information:
• Source address.
The IP address of www.example.com
• Source port number. 80, which is the standard port number for a web server process
• Destination address.
The public IP address of your cable data gateway
• Destination port number. 33333
5. When your cable data gateway receives the incoming message, it checks its session table
for an active session for port number 33333. Finding an active session, the cable data
gateway then modifies the message to restore the original address information replaced by
N
AT. Your cable data gateway sends this reply message to your computer, which
displays the web page from www.example.com. The message now contains the following
address and port information:
• Source address.
The IP address of www.example.com
• Source port number. 80, which is the standard port number for a web server process
• Destination address.
Your computer’s IP address