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IP & network

MTU and MSS calculator

Work out MTU and TCP MSS for any tunnel stack: PPPoE, L2TP, GRE, IPIP, EoIP, VXLAN, WireGuard, OpenVPN and IPsec with exact padding. Commands for Linux, MikroTik, Cisco and a ping test.

    Why sites half-load

    Every tunnel adds headers, leaving less room in a packet. If a large packet does not fit and ICMP “Fragmentation needed” is filtered somewhere, the connection stalls: ping works and small pages load, but big downloads, HTTPS and VPNs hang or drop. The fix is the right tunnel MTU plus TCP MSS clamping.

    MTU and MSS

    MTU is the largest IP packet on an interface. MSS is the largest data payload of a TCP segment: MTU minus 40 bytes for IPv4 (IP and TCP headers) or minus 60 for IPv6. The router rewrites MSS in SYN packets so both ends send segments that fit.

    Ready tunnel configs

    The WireGuard MTU is already handled in the WireGuard generator, PPPoE and MSS for MikroTik in the MikroTik generator. VoIP bandwidth through a tunnel comes from the bandwidth calculator.

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