Kernaali ToolsHow it works
Effective inner MTU equals underlay IP MTU minus added encapsulation bytes. An ICMP echo packet then reserves the IP base header plus eight ICMP bytes before its data.
Worked example
An underlay MTU of 1500 with a 60-byte allowance leaves 1440 bytes for the inner IP packet. IPv4 ICMP echo data fits in 1412 bytes without options.
Inputs, edge cases & common mistakes
This tool does not probe a host or discover PMTU. A fixed allowance must cover the largest overhead in use, including variable ESP padding. Ethernet headers do not usually count toward the configured IP MTU. IPv6 minimum-link-MTU requirements and fragmentation rules require separate review.
Technical references
Reviewed 9 September 2026. Found an issue? Send a correction with a reproducible example.