Port already in use but nothing in netstat

It was a bug I faced something like year ago, and I didn’t fixed that in the past. This time issue was exactly the same (though app was different).

The problem

ng serve
Port 4200 is already in use. 
Use '--port' to specify a different port.

ok let’s check whether anything is blocking the port.

Let’s call netstat in cmd with admin rights:

netstat -ano | findstr 4200

there is no service using that port… strange.

I was close to surrender again, and change port of the app, but I started googling and meet this great article:

As it turned out there Windows makes port reservations in range 1025-65534. This is especially visible if you’re working in configuration: Win 10 + Hyper-V + Docker for Windows.

How to fix it?

Check whether your port is in excluded range by running:

netsh int ipv4 show excludedportrange tcp

Protocol tcp Port Exclusion Ranges

Start Port    End Port
----------    --------
      4137        4367
      9800        9800
      9801        9801
     49671       49770
     61902       62001

if your port is in that range, best solution would be to :

1. Disable Hyper-V

dism.exe /Online /Disable-Feature:Microsoft-Hyper-V

2. Reserve the port for your app

netsh int ipv4 set dynamic tcp startport=4200 numberofports=1

3. Enable Hyper-V

dism.exe /Online /Enable-Feature:Microsoft-Hyper-V /All

Some people need to run also that command

reg add HKLM\SYSTEM\CurrentControlSet\Services\hns\State /v EnableExcludedPortRange /d 0 /f

as described in this github issue .