Netgear 13300241 用户手册
Advanced Settings
52
WiFi Cable Modem Router C3700
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 the public IP address of your modem router. This
step 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 that is chosen by the modem router,
such as 33333. This step 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 that is 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
. The IP address of your computer.
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 traf fic is no longer accepted on port number 33333.
Port Triggering to Open Incoming Ports
In the preceding example, requests are sent to a remote computer by your modem router
from a particular service port number, and replies from the remote computer to your modem
router are directed to that port number. If the remote server sends a reply to a different port
number, your modem router does not recognize it and discards it. However, some application