TCPView
TCPView is a Windows program that can be useful to check which process is using which port. When troubleshooting, this can be especially useful to check why a certain process (e.g. SLNet, Cassandra, etc.) is not able to listen on a specific port.
To use this tool:
Download the tool from the Microsoft website.
Extract the zip file and run Tcpview.exe as Administrator.
The tool will then display an overview of the various processes in your system with the port they are using.
To free up a specific port, right-click the process using the port and select Close Connection.