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Using the router to find TCP/UDP ports

Here's a quick tip to help you figure out which TCP/UDP ports are being used by an application.

To find out what TCP/UDP ports are being used by an application, simply make up a temporary access-list (ACL) and debug it on your router. For example, to find out what ports PCANYWHERE uses, make a temporary ACL and debug it.
Example:

access-list 199 permit ip host 123.123.123.123 any
and add another line that says "access-list 199 permit ip any host 123.123.123.123."
Then run the following command: "debug ip packet detail 199." Make sure you clear IP routes and cache and do a term mon before debugging and you will see the ports 5631 and 5632 for PCANYWHERE. Just make sure to do a u all to disable all debugging when finished.

This was last published in May 2001

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