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Dev.to
6/22/2026
IPv6 NAT66 Behind a FritzBox: The RouterOS 7 Bug That Broke WiFi Clients

IPv6 NAT66 Behind a FritzBox: The RouterOS 7 Bug That Broke WiFi Clients

Short summary

RouterOS 7 bug causes FritzBox WiFi clients to lose IPv6 connectivity when MikroTik runs NAT66 masquerading: RouterOS 7 enables router advertisements on the WAN interface by default, causing packets to route through MikroTik unexpectedly and get dropped. The fix constrains NAT66 source addresses to internal ULA ranges and uses firewall rules to prevent transit traffic.

  • •RouterOS 7 enables Router Advertisement on all interfaces including WAN by default
  • •This routes FritzBox WiFi clients' IPv6 traffic through MikroTik, triggering firewall drops
  • •Solution: constrain NAT66 to fd00::/8 ULA range and add strict firewall forward rules

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