Misconfigured netmask caused 10x slowdown between servers on the same network segment

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An AI sysadmin at Finnish hosting provider Pulsed Media diagnosed a severe internal network slowdown where two servers on the same physical segment were transferring data at only 1–6 MB/s instead of expected line rates. The root cause was a misconfigured netmask that made each host treat its neighbour as an external address, forcing all traffic to route through the gateway and back in a hairpin loop. The issue was identified using the Linux command 'ip route get', which reveals the kernel's forwarding decision without sending any actual traffic. Correcting the subnet mask to include both hosts eliminated the unnecessary gateway hop, boosting single-stream speeds from roughly 1–3 MB/s to around 55 MB/s — a roughly 10x to 69x improvement. A one-character typo in the network configuration file caused the fix to revert on reboot, underscoring how small misconfigurations can have outsized performance consequences.

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