Some Websites Work, Others Don't? VPN Connects But No Traffic? Fix MTU Issues

The Weirdest Internet Problem: Some Sites Work, Others Don't

Your internet is working. You can Google things. YouTube loads. Reddit works fine. But your bank website won't load. Your work VPN connects but then nothing works through it. Gmail is fine but you can't access certain websites.

Speed test shows good speeds. Ping works. You're definitely connected. But specific sites just timeout or hang forever.

This bizarre symptom pattern screams MTU problem (Maximum Transmission Unit). Your network is sending packets that are too large, getting fragmented or dropped, and breaking specific websites while leaving others working fine.

What MTU Actually Is (Simple Version)

MTU is the maximum packet size your network allows. Think of it like maximum box size for shipping:

• Standard MTU: 1500 bytes (works everywhere)
• Your network MTU: Maybe set to 1500
• But somewhere in the path: A device only handles 1400 bytes
• Result: Large packets get dropped, small packets work fine

Why some sites work and others don't:

• Google homepage: Small packets, loads fine
• Banking site with heavy security: Large packets, gets dropped
• Plain email: Small packets, works
• Email with attachments: Large packets, fails

How to Know If MTU Is Your Problem

Classic MTU problem symptoms:

1. VPN connects but nothing works through it

- VPN connects successfully - But web browsing, apps all timeout - This is the #1 MTU symptom

1. Some websites load, others don't

- Simple sites: work fine - Complex sites with HTTPS/security: fail - Pattern is consistent (same sites always fail)

1. Can ping but can't browse certain sites

- Small ping packets: work - Large web packets: fail

1. Works on mobile data but not WiFi (or vice versa)

- Different networks have different MTU limits - Problem appears only on one network

1. SSH/remote desktop connects but hangs during use

- Initial connection: small packets, works - Data transfer: large packets, fails

Quick MTU Test (Windows)

Test if your MTU is causing issues:

ping google.com -f -l 1472

What this does: Sends 1472 byte packet that can't be fragmented

Good result:

Reply from 142.250.80.46: bytes=1472 time=15ms

MTU is fine.

Bad result:

Packet needs to be fragmented but DF set

MTU problem confirmed.

If it fails, test smaller sizes:

ping google.com -f -l 1464
ping google.com -f -l 1400
ping google.com -f -l 1352

Find the largest size that works. That's your actual MTU limit (add 28 to the number).

Quick MTU Test (Mac/Linux)

ping -D -s 1472 google.com

Mac alternative:

ping -D -s 1464 google.com

Same logic: find largest packet size that doesn't get dropped.

How to Fix MTU Issues

Fix 1: Lower MTU on Your Network Adapter (Windows)

Step 1: Open Command Prompt as Administrator

Step 2: Find your network interface name:

netsh interface ipv4 show subinterfaces

Step 3: Set MTU to 1400 (safe value):

netsh interface ipv4 set subinterface "Wi-Fi" mtu=1400 store=persistent

(Replace "Wi-Fi" with your interface name)

Step 4: Test if websites now work

If 1400 works, you can try increasing:

• Try 1450, test
• Try 1480, test
• Find sweet spot where everything works

Fix 2: Lower MTU on Mac

Step 1: Find your network interface:

networksetup -listallhardwareports

Step 2: Set MTU:

sudo networksetup -setMTU Wi-Fi 1400

Step 3: Verify:

networksetup -getMTU Wi-Fi

Fix 3: Lower MTU on Linux

Temporary (until reboot):

sudo ip link set dev eth0 mtu 1400

Permanent (add to /etc/network/interfaces):

iface eth0 inet dhcp
    mtu 1400

Fix 4: Router MTU Setting (Affects All Devices)

Better solution: Fix it at router level so all devices benefit.

1. Login to router (192.168.1.1 or similar)
2. Find "MTU" setting (usually in WAN or Advanced settings)
3. Change from 1500 to 1400 or 1450
4. Save and reboot router

Common locations:

• TP-Link: Network > WAN > MTU Size
• Netgear: Advanced > Setup > WAN Setup > MTU Size
• ASUS: WAN > Internet Connection > MTU
• Linksys: Connectivity > Internet Settings > MTU

VPN-Specific MTU Fix

If VPN connects but nothing works:

Most VPN software has MTU settings.

OpenVPN: Add to config file:

mssfix 1400
tun-mtu 1400

WireGuard: Set MTU in interface config:

MTU = 1400

Commercial VPN apps:

• Look for "MTU" in advanced settings
• Try values: 1400, 1420, 1450
• Test until it works

Why MTU Problems Happen

Common causes:

1. PPPoE connections (DSL internet)

- PPPoE adds 8 byte overhead - Standard 1500 MTU becomes 1492 effective - Some devices don't handle this well

1. VPN/tunnels

- Encryption adds overhead - 1500 MTU packets become too large when encrypted - Need lower MTU for VPN traffic

1. ISP network equipment

- Some ISPs use MTU < 1500 internally - Don't advertise this to customers - Causes mysterious failures

1. Multiple layers of tunneling

- VPN through VPN - VPN over PPPoE - Each layer adds overhead

How to Find Optimal MTU

Formula: Optimal MTU = (Largest working ping packet size) + 28

Example:

• ping -f -l 1464 works
• ping -f -l 1472 fails
• Optimal MTU = 1464 + 28 = 1492

Common safe MTU values:

• 1500: Standard, works most places
• 1492: PPPoE connections
• 1450: Safe for most VPNs
• 1400: Very conservative, works almost everywhere
• 1280: Minimum for IPv6, always works but inefficient

The Bottom Line

If some websites work but others don't, or VPN connects but traffic doesn't flow, you probably have an MTU problem.

Quick diagnosis:

ping google.com -f -l 1472

If this fails, MTU is your issue.

Quick fix: Lower MTU to 1400 on your network adapter or router. Test if problems disappear.

Optimal fix: Test different MTU values (1400, 1450, 1480) to find highest value that works reliably. Use that.

Where to set it:

• Router: Affects all devices (best option)
• Network adapter: Affects just one device
• VPN software: Only affects VPN traffic

Most people never encounter MTU issues. But when you do, the symptoms are bizarre enough that you'll waste hours troubleshooting everything else first. Now you know what to check.