On Networks N300RM User Manual - Manual (Page 51)

Security Settings
51
N300 WiFi ADSL2+ Modem Router (N300RM)
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 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 assigned by the modem router, such
as 33333. This is necessary because two computers could independently be using
the same session number.
Your modem 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
web server then sends this reply message to your modem router.
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 modem router.
Destination port number . 33333.
5. Upon receiving the incoming message, your modem router checks its session table to
determine whether there is an active session for port number 33333. Finding an active
session, the modem router then modifies the message to restore the original address
information replaced by NAT. Your modem router 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.
Destination port number
. 5678, which is the browser session that made the initial
request.
6. When you finish your browser session, your modem router eventually detects a period of
inactivity in the communications. Your modem router then removes the session information
from its session table, and incoming traffic is no longer accepted on port number 33333.