The IPv6 Tunnel That Made the Whole Network Feel Slower

Restoring IPv6 reachability behind a NAT'd home connection via a tunnel broker was the easy part. The mistake that made every device on the network feel sluggish was one boolean flag, and understanding why required being honest about what the tunnel was actually for.

July 30, 2026

This article is also available in Traditional Chinese: 中文版 — same content, just a language difference.

A 6in4 tunnel broker restored globally-routable IPv6 reachability behind a NAT'd home connection with no native IPv6 support. The tunnel itself worked correctly from the moment it was configured. The mistake that made the entire home network feel noticeably slower for every device on it was a single router setting, left at a default that seemed harmless.

Why the tunnel existed at all

The home internet connection sits behind carrier-grade NAT with no native IPv6 of its own — a tunnel broker (a service that carries IPv6 traffic encapsulated inside IPv4 packets to a remote relay, which then routes it onto the real IPv6 internet) was the practical way to get a small number of specific hosts a real, globally-routable IPv6 address despite that. This is worth being precise about, because it shapes the whole rest of the story: the goal was never IPv6 for speed. IPv4 was, and remained, the faster path for essentially all traffic — the tunnel's actual job was giving a couple of specific hosts a stable, publicly reachable address for inbound connections, which NAT alone can't provide.

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