Why Some Websites Won't Load: Understanding MTU and Internet Issues

When your internet connection seems fine but some websites won't load, the culprit is often not the provider or the site itself, but a lesser-known parameter called MTU (Maximum Transmission Unit). Understanding how MTU affects data transmission can help diagnose those mysterious cases where certain sites don't open, VPN connections fail, or downloads are interrupted for no clear reason.

What Is MTU?

MTU stands for Maximum Transmission Unit - it's the largest size a data packet can be to travel through a network segment without being fragmented. In simple terms, MTU defines how many bytes of information fit into a single network packet for a specific segment of your connection.

Each network interface and connection type has its own MTU limit. For traditional Ethernet, the standard value is 1500 bytes - a compromise between efficient data transmission and manageable processing overhead.

When data travels across the internet, it's broken into packets, each containing headers and payload. MTU sets the upper limit for the payload size. If the data is larger than allowed, it must be split into smaller pieces.

It's important to note that MTU isn't a global internet setting. It may vary across:

• the user's computer
• the router
• the ISP
• intermediate backbone segments

If any segment has a lower MTU than expected, data transmission issues can arise.

MTU doesn't directly affect speed. Its role is to ensure packets are delivered correctly. However, an incorrect MTU can lead to data loss, delays, or even failed transmissions, even when the connection appears established.

How MTU Size Impacts Data Transmission

The MTU size determines how data is divided into packets for network transmission. A larger MTU means bigger packets, which reduces header overhead and network load. A smaller MTU requires more packets, each with its own headers.

Problems occur when a device sends packets larger than the next segment allows. Two things can happen: the packet gets fragmented or dropped. Which scenario unfolds depends on network protocols and settings.

Modern networks consider fragmentation undesirable - it increases latency, the risk of data loss, and complicates packet processing. Devices and routers often avoid fragmentation, simply discarding oversized packets and expecting the sender to reduce their size.

If this doesn't happen, the connection behaves unpredictably. Small packets (like requests) go through, but large ones (like website responses or secure handshake data) don't. As a result, only some sites open, with no obvious speed or connectivity errors.

An incorrect MTU doesn't break the internet entirely; it breaks transmission for data above a certain size, making the issue hard to diagnose.

Packet Fragmentation and Its Pitfalls

Fragmentation splits a large data packet into smaller units to pass through a network segment with a smaller MTU. In theory, this should solve size mismatches, but in practice, it often causes errors.

Historically, routers handled fragmentation. If a packet was too big, it was simply split along the way. In modern networks, this approach is considered inefficient and insecure, so fragmentation is often disabled or restricted.

Today, the sender is expected to size packets correctly from the start, using a mechanism called PMTUD (Path MTU Discovery), which identifies the minimum MTU along the route. If PMTUD fails or its messages are blocked, the sender keeps transmitting oversized packets.

This leads to silent packet drops and stalled connections - especially problematic for protocols that can't tolerate large packet loss, such as HTTPS. Visibly, it manifests as "site not loading," even though the connection itself is established.

The most common causes of fragmentation failure include:

• Filtering of ICMP control messages
• Incorrect router configuration
• Tunnels and VPNs
• MTU mismatches between connection types

That's why MTU is a hidden but significant source of network headaches.

Why Websites "Don't Open" Even When Internet Works

MTU issues rarely take down the internet completely. Instead, they cause chaos: some sites load, others hang; messengers connect, but files won't send; VPNs won't establish sessions even though there's internet access.

The reason is that small packets pass through, but larger ones do not. The initial handshake or request is tiny, but responses (especially with HTTPS certificates or large content) require bigger packets.

If those large packets are dropped due to MTU mismatches, the connection freezes while the browser waits for data that never arrives. No clear errors appear, since TCP believes the connection is still active.

This issue is particularly common with:

• HTTPS websites
• VPN connections
• Corporate networks
• Sites outside your region

As a result, MTU problems are often mistaken for a "bad internet" or provider outages when the real culprit is packet transmission settings.

MTU, VPNs, and Strange Network Errors

VPNs almost always reduce the effective MTU because the tunnel adds its own headers, shrinking the usable payload. If this isn't accounted for, packets inside the tunnel become too large.

The result: the VPN connects, but:

• websites don't load
• connections drop unexpectedly
• only some services work
• speed drops dramatically

This is a frequent reason for the classic "VPN is connected, but the internet isn't working." The issue isn't encryption or the server - oversized packets simply don't fit the allowed MTU somewhere along the path.

Similar problems can arise with PPPoE, mobile internet, nonstandard routers, or chains of multiple network devices, each reducing the effective MTU.

Should You Change MTU Manually?

In most cases, you don't need to adjust MTU manually. Modern operating systems and routers are designed to select appropriate values and use PMTUD effectively.

Manual MTU tuning makes sense only if:

• the problem is persistent and reproducible
• the internet only works partially
• VPN or specific websites won't open
• changing your provider or hardware doesn't help

Without understanding the cause, changing MTU may not help and could even worsen the situation. MTU isn't an "internet accelerator" but a compatibility parameter for your network.

Conclusion

MTU is one of those network settings few people think about - until the internet starts acting strangely. It doesn't affect speed directly but determines whether data can reach its destination at all.

An incorrect MTU doesn't break your connection entirely; it creates subtle, hard-to-explain issues where some sites don't load, VPNs fail, and the internet seems unstable for no apparent reason.

Understanding how MTU influences data transmission can help distinguish real network problems from myths about speed or unreliable providers. It's a reminder that even small technical settings can have a big impact on your everyday internet experience.