Netgear AC750 User Manual download pdf (Page 120)

Advanced Settings

120

A750 Wireless Dual Band Gigabit Router JR6150

Port Triggering to Open Incoming Ports

In the preceding example, your router sends requests to a remote computer from a particular

service port number, and replies from the remote computer to your router are directed to that

port number. If the remote server sends a reply to a different port number, your router does

not recognize it and discards it. However, some application servers (such as FTP and IRC

servers) send replies to multiple port numbers. Using the port triggering function of your

router, you can tell the router to open more incoming ports when a particular outgoing port

originates a session.

An example is Internet Relay Chat (IRC). Your computer connects to an IRC server at

destination port 6667.

The IRC server not only responds to your

originating source port, but

also sends an “identify” message to your computer on port 113. Using port triggering, you can

tell the router, “When you initiate a session with destination port 6667, you must also allow

incoming traffic on port 113 to reach the originating computer.”

Using steps similar to the preceding example, the following sequence shows the ef

fects of

the port triggering rule you have defined:

1. You open an IRC client program to start a chat session on your computer.

2. Your IRC client composes a request message to an IRC server using a destination port

number

of 6667, the standard port number for an IRC server process. Your computer then

sends this request message to your router.

3. Your router creates

an entry in its internal session table describing this communication

session between your computer and the IRC server. Your router stores the original

information, performs Network Address Translation (NAT) on the source address and port,

and sends this request message through the Internet to the IRC server.

4. Noting your port triggering rule and having observed the destination port number of 6667,

your router creates

an additional session entry to send any incoming port 113 traffic to your

computer.

5. The IRC server sends a return message to your router using

the NAT-assigned source port

(as in the previous example, say port 33333) as the destination port. The IRC server also

sends an “identify” message to your router with destination port 113.

6. When your router receives

the incoming message to destination port 33333, it checks its

session table for 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. When your router receives

the incoming message to destination port 113, it checks its

session table and finds 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 must know which inbound ports the

application needs. Also,

you must know the number of the outbound port that triggers the opening of the inbound

ports. You can usually find this information by contacting the publisher of the application or

user groups or newsgroups.

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