Use NETSTAT to find out what application is using what port

Netstat is a very useful application that I use frequently for finding listening ports. On occasion I'll find something that is consuming a wide range of listening ports. For example, I've seen time and time again where Microsoft's DNS server, if not properly setup, consuming all UDP listening ports. Here's a tip to find out what application is occupying what listening port(s).

Open command prompt or terminal window and type the following,

netstat -h

This will list all available options for netstat. Older versions of netstat on Windows machine will not include all options mentioned in this article. I usually use options -an, example,

netstat -an

This will list all ports, listening or not, and what interface the port is bound to. When you run netstat -an you may see UDP listening ports scroll off the screen. To find out what application is using these ports type the following (windows computer),

netstat -anb

The -b option will also list the applications responsible for opening the listening port and hopefully help in your troubleshooting efforts. In my example of Microsoft's DNS Server I typically ask if the DNS server is being used and if not stop the service and set it to disabled, this immediately clears all listening UDP ports by application dns.exe.