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Sometimes I get a message, 'port already open." What does it mean?

Sometimes I get a message port already open, when I try to connect to the Internet. What does it mean? How can I know which application is already using the net. I tried using task manager, but which one I couldn't tell.

The message received by your program or operating system indicated that a newly launched application is try to open a port that is already in use.

All programs that are network aware use IP Addresses and ports in order to communicate and transfer data. When a service runs on a server (i.e. Web server.) This service is bound to the IP Address of that machine which is, and "listens" for incoming connections on a specific port (80).

Do you have questions about networking, VPN security or VoIP? Then visit, one of the few websites recommended by Cisco Systems in its world class Cisco Academy program.

Now, in order to resolve your problem, you need to identify which application is using the port for which your system is complaining. This can be easily done by downloading one of them personal firewall products (Zone Alarm is freeware and recommended) which will give you the ability to control which programs are allowed to initiate connections to the Internet.

If you would like to see what ports are being used, or listening for incoming connections, you can open your command prompt and type "netstat –a" and get a listing of the ports being used.

Alternatively, there is a freeware program I recently discovered called fport v2.0 ( that will show you the same results.

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This was first published in September 2003

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