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In page Port triggering:

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When connecting to IRC, the client computer typically makes an outgoing connection on port 6667 (or any port in the range 6660–7000), causing the IRC server to attempt to verify the username given by making a new connection back to the client computer on port 113. When the computer is behind a NAT, the NAT silently drops this connection because it does not know to which computer behind the NAT it should send the request to connect. These two transport-level connections are necessary for the application-level connection to the IRC server to succeed (see Internet protocol suite). Since the second TCP/IP connection is not possible, the attempted connection to the IRC server will fail.