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Find the largest packet size that survives the path.
The MTU Probe tool discovers the maximum transmission unit — the largest IP packet size — that can travel end to end between our probe and a destination without being fragmented or dropped.
Enter a hostname or IP address and run the tool. The probe sends progressively larger packets with the Don't-Fragment flag set to find the largest size that survives the path.
Input: Hostname or IP — e.g. edge.example.org.
The result reports the discovered path MTU in bytes and the largest probe that succeeded. A path MTU lower than 1500 indicates a tunnel, VPN, or PPPoE link with a reduced frame size — the cause of black-hole fragmentation issues where large packets silently disappear.
→ Diagnose large-file transfers that stall while small packets work.
→ Find the right MSS clamping value for a VPN or tunnel.
→ Confirm a PPPoE broadband link has the expected 1492 MTU.
→ Troubleshoot IPsec or GRE tunnel performance problems.
1500 bytes is standard Ethernet. PPPoE broadband links reduce it to 1492, and many VPN tunnels and overlays (GRE, WireGuard, IPsec) drop it to 1400 or lower. Anything below 1500 on a plain path suggests a tunnel somewhere.
When a link cannot carry a 1500-byte packet and the Don't-Fragment bit is set, the oversized packet is silently dropped. Large transfers then stall while small ones (like pings) work fine — a classic MTU black hole. Clamping MSS or lowering interface MTU fixes it.
A common safe value is 1360 for paths crossing VPNs/tunnels, or 1452 for PPPoE. MSS clamping is usually preferable to lowering the interface MTU because it avoids fragmenting non-TCP traffic.