This is an FYI posting: I have been using Norton 360 Deluxe’s VPN for several years.
I recently switched from an old copper wire internet service provider to a newer wireless internet service provider here in the U.S. (T-Mobile). Immediately, the Norton VPN began to fail. Internet research confirmed VPN failure to be a frequent problem with this ISP. The problem for me was solved by determining the Windows pre-set MTU setting (1500 for me), which was causing fragmentation of the sent packets, which in turn was involved with the Norton VPN not working as intended, via this wireless ISP. I permanently reset my PC’s MTU setting to a lower level (1460 for me) which stopped the fragmentation and allowed the Norton VPN to work again.
Internet searching on the subject (for instance, “VPN does not work with T-Mobile”) came up with links provided by other users who had the same problem and how they solved it. This Norton Community Forum may not have the room to define all the steps needed and taken to fix the MTU problem. I am a lower-level “techie”, yet still could successfully follow the instructions posted by others. Again, it’s suggested to do an internet search on the subject for resolution if Norton’s VPN is failing for you while using the above-mentioned ISP. Internet-posted instructions solved my problem.
The issue with Norton VPN not working with T-Mobile wireless ISP service is a known compatibility problem, primarily caused by T-Mobile’s network configuration and a technical setting called the MTU (Maximum Transmission Unit) size. The VPN can be made to work by adjusting settings on your device.
Primary Cause: MTU Discrepancy
T-Mobile’s network often uses a different MTU size (around 1450 bytes) compared to the standard 1500 bytes used by most internet providers and VPN software, including Norton VPN. This difference causes data packet fragmentation, which disrupts the VPN connection and makes it fail.
Solutions and Troubleshooting Steps
Here are the primary ways to fix the problem:
Adjust the MTU Setting (Recommended Fix)
The most effective solution is to manually change the MTU setting on your computer or device to a lower value that is compatible with T-Mobile’s network, such as 1460 or 1400 bytes. You can find specific instructions online for how to do this for your specific operating system (Windows, Mac, etc.) by searching for “how to change MTU settings on [your operating system]”.
Change the VPN Protocol
Switching the connection protocol within the Norton VPN application settings can also resolve the issue.
Open the Norton VPN app settings.
Look for an option to change the VPN Protocol.
Experiment with different available protocols, such as OpenVPN (TCP) or the proprietary Mimic protocol if available, as they handle data transmission differently.
General Troubleshooting Steps
If adjusting the MTU and protocol doesn’t work, try these general steps:
Verify Internet Connection: Ensure your T-Mobile internet is working properly without the VPN connected.
Update Software: Make sure your Norton VPN app and device’s operating system are running the latest updates.
Restart Devices: Restart both your T-Mobile gateway (router) and the device you’re trying to use the VPN on.
Temporarily Disable Firewall/Antivirus: Occasionally, the built-in firewall of your device might conflict with the VPN. Temporarily disable it to see if the VPN connects, then reconfigure the firewall settings to allow the Norton app.
Reinstall Norton VPN: Uninstalling and then reinstalling the Norton VPN app can fix any corrupted installations or misconfigurations.
Contact Support: If the problem persists, contact Norton Support for further assistance, as they may have specific server recommendations or advanced troubleshooting for T-Mobile users.
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Finding and changing your MTU (Maximum Transmission Unit) setting generally involves using the command line on a computer. The process depends on your device’s operating system. Note that you generally cannot change the MTU settings on the T-Mobile gateway itself or on iOS/Android mobile devices.
For Windows 10/11
You can find and change the MTU setting using the Command Prompt with administrator privileges. A suggested MTU value for T-Mobile is 1460.
Open Command Prompt as administrator by typing cmd in the search bar, right-clicking, and selecting “Run as administrator”.
Find your network interface name and current MTU by typing netsh interface ipv4 show subinterfaces.
Set a new MTU value (e.g., 1460) using the command netsh interface ipv4 set subinterface "[interface name]" mtu=[MTU value] store=persistent, replacing [interface name] and [MTU value] as needed.
Verify the change by running netsh interface ipv4 show subinterfaces again.
For macOS
Use the Terminal to check and modify the MTU setting.
Open Terminal from Applications > Utilities.
Find your network service name and current MTU using networksetup -getMTU [network service name].
Set a new MTU value with sudo networksetup -setMTU "[network service name]" [MTU value], entering your administrator password when prompted.
For Mobile Devices and Routers
The MTU size on T-Mobile gateways is fixed at 1500 and cannot be changed manually.
iOS/Android devices typically lack built-in options to modify the MTU without complex and potentially risky modifications.
If you are using a mobile device, adjusting the MTU on the computer you are connecting with or trying other troubleshooting steps like changing the VPN protocol in your Norton settings may be more effective.