Fix FWP_E_INVALID_NET_MASK (0x8032001F) in Windows Firewall

1. Your DHCP server is handing out a bad subnet mask

The most common trigger for FWP_E_INVALID_NET_MASK (0x8032001F) is a misconfigured DHCP server pushing an invalid or zero-length subnet mask. I've seen this happen most often on home networks where a router's DHCP pool uses something like 255.0.0.0 for a /24 LAN, or a VPN tunnel interface inherits a mask like 0.0.0.0.

What's actually happening here is that Windows Filtering Platform (WFP) checks every network interface's subnet mask against firewall rules that reference that interface. If the mask is malformed — all zeros, non-contiguous bits, or a value outside valid CIDR ranges — WFP throws this error instead of applying the rule silently.

How to fix it

1. Open a Command Prompt as Administrator.
2. Run ipconfig /all and check every adapter's subnet mask. Look for anything that isn't a standard mask (e.g., 255.255.255.0, 255.255.0.0).
3. If you see a mask with a zero octet in the wrong position (like 255.0.255.0) or 0.0.0.0, that's your problem.
4. Try renewing the IP lease: ipconfig /release then ipconfig /renew.
5. If the bad mask comes back, you need to fix your router's DHCP settings. Log into your router, go to LAN/DHCP settings, and set the subnet mask to match your network. For a typical home network, that's 255.255.255.0.

The reason step 4 works temporarily is that /renew makes the DHCP server send a fresh lease. If the server's config is wrong, you'll just get the same bad mask again — so step 5 is the real fix.

2. A third-party VPN or network filter software left a corrupted rule

Second most common cause: software like NordVPN, ExpressVPN, or even old Cisco AnyConnect clients create WFP rules that reference a subnet mask format WFP no longer accepts after a Windows update. I've debugged this on Windows 10 22H2 and Windows 11 23H2 specifically.

The corruption usually happens when the VPN client uninstalls improperly — it removes the virtual adapter but leaves the firewall rule pointing to an interface with a now-invalid mask. WFP then fails to apply the rule and logs 0x8032001F.

How to fix it

1. Open Windows Security -> Firewall & network protection -> Advanced settings.
2. In the left pane, click Inbound Rules and scan for rules from your VPN provider. They're usually named something like "NordVPN Kernel Mode Filter" or "Cisco AnyConnect Firewall Rule".
3. Right-click and disable them one by one. If the error stops, you've found the bad rule. Delete it.
4. If you can't find the rule visually, use PowerShell: Get-NetFirewallRule | Where-Object { $_.DisplayName -match "VPN" } | Remove-NetFirewallRule — but back up your firewall config first with netsh advfirewall export "C:\backup.wfw".

Skip the step about reinstalling the VPN — that often re-imports the same broken rule. The real fix is to nuke the rule and let the software recreate it fresh next time you connect.

3. Your own custom firewall rule has a bad subnet mask

If you've manually created firewall rules with netsh advfirewall or PowerShell and you specified a subnet mask in the wrong syntax, you'll hit this error. The format WFP expects is IP/mask (CIDR notation), not IP, mask or IP subnetmask.

I've seen people write 192.168.1.0 255.255.255.0 instead of 192.168.1.0/24 and get 0x8032001F. WFP isn't forgiving about this.

How to fix it

1. List all your custom rules: netsh advfirewall firewall show rule name=all verbose.
2. Look for rules with LocalAddress or RemoteAddress entries that don't use CIDR. An invalid mask looks like 192.168.1.0/255.255.255.0 — that's wrong. It should be 192.168.1.0/24.
3. Delete the bad rule: netsh advfirewall firewall delete rule name="Your Rule Name".
4. Recreate it with correct CIDR: netsh advfirewall firewall add rule name="Your Rule Name" dir=in action=allow remoteip=192.168.1.0/24.

The reason step 4 succeeds is that WFP internally converts CIDR notation to a binary mask. If you pass a decimal mask in the wrong position, it's parsed as a different value — and WFP's validation catches it and returns 0x8032001F. CIDR notation avoids that ambiguity entirely.

Quick-reference summary

If none of these work, the nuclear option is resetting Windows Firewall entirely: netsh advfirewall reset. That clears everything — all rules, all filters — and starts fresh. You'll lose any custom rules though, so export first.