Data on the internet is not sent as one continuous stream. It is cut into packets, and every network along the way has a limit on the largest packet it can pass on. That limit is the MTU: maximum transmission unit.

Most of the time nobody needs to think about it. Devices and routers agree on it themselves. But when that agreement fails, the symptoms are among the most confusing in home networking.

The simple picture

Imagine a road used by lorries, with one low tunnel in the middle. Small lorries pass without trouble. A lorry that is too tall must split its load in two, or turn back.

On the internet, that splitting is called fragmentation, and modern networks often refuse to do it. Instead they send a message back to the sender: "your packet is too big". The trouble starts when that reply is blocked along the way, by an over-strict firewall, for instance. The sender never finds out, keeps sending large packets, and they keep disappearing.

The telltale symptoms

What makes MTU problems hard to spot is how selective they are:

  • Some sites open normally; others spin without ever loading.
  • A page starts to appear, then stops halfway, usually when a large part such as an image or script loads.
  • Sites and apps using encrypted connections, banking, work email, are hit more often.
  • Short chat messages send, but photos don't.
  • The problem appeared only after installing a VPN, changing router, or switching connection type.

Small packets get through, large ones vanish. That is why the connection seems to "almost work".

Why PPPoE is often involved

Many home connections use PPPoE, which adds an 8-byte label to every packet. The room for data shrinks from 1500 to 1492. A correctly configured router handles this automatically. A router set up by hand with an MTU of 1500 over PPPoE is one of the most classic sources of trouble.

VPNs do something similar with larger labels, shrinking the data room further.

Checking without guessing

On a Windows computer, ping can send a packet of a set size with fragmentation forbidden:

  • Open Command Prompt and run ping google.com -f -l 1472.
  • If it reports that the packet needs to be fragmented, lower the number step by step, 1464, 1452, and so on , until it succeeds.
  • The largest number that succeeds, plus 28, is the path's MTU. 1472 + 28 = 1500.

On macOS and Linux the equivalent command uses different options, but the principle is the same.

Fixing it

If the measured value is smaller than the router's setting, change the MTU in the router's WAN settings to the figure you found. On PPPoE connections, many routers also have an option called MSS clamping that is worth switching on, it tells devices behind the router to use the right packet size from the start.

If the problem only happens with the VPN on, change the MTU in the VPN app's settings, not on the router.

Some sites refusing to load can also stem from DNS or blocking. Other checks are in the guide to specific apps that won't open.

MTU is the largest packet that can pass. When it is wrong, small packets get through and large ones vanish, so some sites won't load. Measure with sized pings, then set the value on the router or VPN. As long as there are no symptoms, leave it on automatic.

Frequently asked questions

What is the right MTU value?

For ordinary Ethernet, 1500. PPPoE connections usually need 1492. VPNs and some tunnels need smaller values still. On most home routers the automatic setting is already correct and needs no change.

Does lowering MTU make the internet faster?

No. The right MTU only prevents problems. Lowering it more than necessary slightly adds overhead, because data is split into more packets.