Netgear DC112A User's Guide download pdf (Page 60)

Control Internet Access

AirCard Smart Cradle

Application Example: Port Triggering for Internet Relay Chat

Some application servers, such as FTP and IRC servers, send replies to multiple port

numbers. Using port triggering, you can tell the cradle to open more incoming ports when a

particular outgoing port starts 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 cradle, “When you initiate a session with destination port 6667, you must also allow

incoming traffic on port 113 to reach the originating computer.” The following sequence shows

the effects of this port triggering rule:

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 cradle.

3. Your cradle creates an entry in its internal session table describing this communication

session between your computer and the IRC server. Your cradle 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 observing the destination port number of 6667, your

cradle creates another session entry to send any incoming port 113 traffic to your computer.

5. The IRC server sends a return message to your cradle using the NAT-assigned source port

(for example, port 33333) as the destination port and sends an “identify” message to your

cradle with destination port 113.

6. When your cradle receives the incoming message to destination port 33333, it checks its

session table to see if a session is active for port number 33333. Finding an active session,

the cradle restores the original address information replaced by NAT and sends this reply

message to your computer.

7. When your cradle 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

cradle 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 cradle eventually senses a period of inactivity in the

communications. The cradle then removes the session information from its session table

and incoming traffic is no longer accepted on port numbers 33333 or 113.

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Netgear DC112A User's Guide Page 60