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VPN MTU test and advisor

Browsers cannot perform reliable Path MTU Discovery. This tool calculates a conservative starting point from the link MTU, protocol and IP family, labels it as an estimate, and explains how to confirm the value with operating-system probes.

Calculate a starting MTU

No packet probe has run. This is an advisor, not a Path MTU Discovery probe.

Fragmentation and black-hole symptoms

  • Small pages load but large uploads or downloads stall.
  • A tunnel connects, yet some HTTPS sites time out during TLS.
  • Video or calls work briefly and then freeze on a path that blocks ICMP.

Confirm with your OS using progressively smaller ping payloads and the do-not-fragment option where supported. OpenVPN can use mssfix or tun-mtu; WireGuard commonly starts near 1420 on a 1500-byte link, but nested tunnels, PPPoE and IPv6 can require less.

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Frequently asked questions

Is this real Path MTU Discovery?
No. The browser cannot send the required controlled ICMP or UDP probes. The result is an explicit estimate based on header overhead.
Why can the correct MTU differ by network?
PPPoE, mobile carriers, nested VPNs, IPv6 and intermediate tunnels change the available payload size.
Should I choose the smallest possible MTU?
No. An unnecessarily small MTU adds overhead. Start conservatively, test, then increase only while the path remains reliable.