Whenever an application wants to make itself accessible over the
network, it claims a TCP/IP port, which means that port can’t be used by
anything else. So if you need to use an in-use port, how do you tell
what application is holding it?
There’s a number of ways to tell what application has the port
locked, but we’ll walk through the built-in way using the command line
and Task Manager, and then a great freeware application that does it all
in one utility.
Use Built-In Tools to See What is Listening on a Port
The first step is to use a command-line tool to see what ports are in
use, and use a special flag that tells us which port is assigned to
each Windows process identifier number. Then we can use that number to
look up exactly which process it is.
Open up a command prompt and type in the following—you may have to open in Administrator mode to see all processes:
netstat -aon | more
If you look on the right-hand side, you’ll see where I’ve highlighted
the list of PIDs, or Process Identifiers. Find the one that’s bound to
the port that you’re trying to troubleshoot—for this example, you’ll see
that 0.0.0.0:80, or port 80, is in use by PID 4708.
Now you can simply open up Task Manager—you might have to use the
option to Show Processes for All Users, and then you’ll be able to find
the PID in the list. Once you’re there, you can use the End Process,
Open File Location, or Go to Service(s) options to control the process
or stop it.
Use CurrPorts to View What is Listening
If you aren’t really the command-line type, or you’d rather just use a
simple utility to do it all in one step, you can use the excellent
freeware CurrPorts utility by NirSoft, one of our favorite freeware
authors.
Just open up the utility, and you can see everything—processes, ports, remote ports, the full path to the process.
If you double-click on any of the items, you’ll see every single detail.
You can also use CurrPorts to directly kill the process if you want.
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