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Port 546 is reserved for the client-side of the Neighbor Discovery Protocol (NDP). This is used for communication between IPv6 nodes (such as a device and its router) to discover and configure network information (such as IP address). Router Advertisement (RA) messages sent by routers use port 547 (router-side), and devices use port 546 to receive these messages. See https://forum.cloudron.io/topic/13566/infomaniak-ipv6-issues/61