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Determine State of a Port Using Telnet

Using telnet, you can make an educated guess whether a port on a remote host is open, closed or blocked by a firewall by the error message it throws.

If you get no error message, the port is open.

If you get an error like the following:

Unable to connect to remote host: No route to host


you are almost certainly seeing a blocked (by firewall or whatever) port.

If you get an error like the following:

Unable to connect to remote host: Connection timed out


the port is almost certainly a closed port.

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