When starting a service, you may occasionally encounter a situation where a port is occupied. However, when you actually query it using netstat -ano | findstr :[prot], you might find that no program is occupying it. This is usually because it has been occupied by Windows as a reserved port.

If a program is indeed occupying it, you can use the taskkill command to terminate the process.

Generally, simply changing to another port will solve the issue. However, when you want to have control over your ports, you need to prevent Windows from using these ports as reserved ports.

Viewing Reserved Ports

Open CMD or PowerShell as an administrator and use the following command to view them:

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netsh interface ipv4 show excludedportrange protocol=tcp

The command will list all currently reserved ports.

Random Refresh

Although the ports are randomly refreshed upon every reboot, you can restart the service using the following commands to refresh the ports without rebooting:

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net stop winnat
net start winnat

Then, check again to see if the port you want is still occupied.

Adding Exclusion Rules

However, to stably use a preferred port, it is more reliable to add an exclusion rule. First, stop the WinNAT service:

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net stop winnat

Then, declare your preferred port range:

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netsh int ipv4 add excludedportrange protocol=tcp startport=7650 numberofports=10

This command excludes 10 ports starting from 7650, which is 7650~7659. Then, simply restart the WinNAT service:

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net start winnat

Removing Exclusion Rules

When you no longer need them, also stop the service first, and then use the command to delete the exclusion:

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netsh int ipv4 delete excludedportrange protocol=tcp startport=7650 numberofports=10

Finally, restart the service:

Cause

The root cause of this issue is usually that Windows has Hyper-V or WSL2 enabled. Windows utilizes a service called WinNAT to dynamically allocate a range of ports as dynamic ports for these virtual machines and containers.

Unfortunately, this range is usually very large (for example, potentially anywhere from 1024 to 50000), and it is randomly allocated every time the computer restarts. This leads to a situation where a port that worked fine yesterday might be occupied by the system today and become unusable.