Scanning a /64 or even a /56 (which you ought to get from your provider) is infeasible.
Nevertheless, you have the exact same problem already today with ipv4. Just with a NAT inbetween, which is usually replaced with a firewall for ipv6. Also, when a specific device opens up a port via upnp it better does this on purpose.
Nevertheless, you have the exact same problem already today with ipv4. Just with a NAT inbetween, which is usually replaced with a firewall for ipv6. Also, when a specific device opens up a port via upnp it better does this on purpose.