// Tool
Path MTU Discovery
Browsers can't measure your MTU directly — they have no raw socket access. This tool walks you through the real test: a binary-search ping sweep with the Don't Fragment flag set. Pick your OS, run the suggested command, click whether it worked, and the tool narrows in on your actual path MTU.
Pick a host that reliably replies to ICMP. Cloudflare's 1.1.1.1 and Google's 8.8.8.8 are good defaults.
Did the ping reply normally?
What does failure look like?
Linux / macOS:
Windows:
PMTU black holes: sometimes the larger ping just hangs with no reply. Treat any timeout as a failure — the binary search still converges.
ping: sendto: Message too long or frag needed and DF set; or all packets time out.Windows:
Packet needs to be fragmented but DF set.PMTU black holes: sometimes the larger ping just hangs with no reply. Treat any timeout as a failure — the binary search still converges.
Path MTU: bytes
Common MTU reference values
| MTU | Profile |
|---|---|
| 9000 | Jumbo frames (data center) |
| 1500 | Standard Ethernet — most wired and Wi-Fi networks |
| 1492 | PPPoE (DSL) |
| 1480 | GRE tunnel |
| 1460 | PPTP tunnel |
| 1438 | IPsec ESP (common) |
| 1428 | IPsec ESP + ESN |
| 1420 | WireGuard (default) |
| 1400 | OpenVPN (common) |
| 1280 | IPv6 minimum (RFC 8200) |
Frequently Asked Questions
Why can’t a website measure my MTU directly?
Browsers do not have raw socket access and cannot set the IP Don’t Fragment flag, so they cannot run a real path-MTU probe. Accurate MTU testing has to use
ping with the DF bit set, which only the operating system can do — which is what this tool walks you through.How does this tool actually find my MTU?
It guides you through a binary search using
ping with the DF flag set. Each round narrows the largest payload size that gets through without fragmentation. Your path MTU is that payload size plus 28 bytes (20-byte IPv4 header + 8-byte ICMP header).Why is the result payload + 28 instead of just the payload?
The size you pass to
ping -s is the ICMP data only. Add 8 bytes for the ICMP header and 20 bytes for the IPv4 header to get the actual MTU. For IPv6 the math is payload + 48 (40-byte IPv6 header + 8-byte ICMPv6 header).What MTU is normal?
Standard Ethernet is 1500. PPPoE drops it to 1492. WireGuard tunnels typically land at 1420 and IPsec around 1438. The IPv6 minimum is 1280. Anything else usually means stacked tunnels or a network with reduced MTU.
Why does the ping sometimes hang instead of returning an error?
Some networks silently drop oversized packets without sending back the ICMP "fragmentation needed" message PMTUD depends on — a PMTU black hole. Treat any timeout as a failure and keep probing smaller; the binary search still converges.
