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I've been trying quite unsuccessful to exclude certain clients from getting an IPv6 via DHCP in a network. Since I just wanted to exclude specific interfaces I've used the MAC address instead of the DUID – the “DHCP unique identifier”, see DHCPv6. As it turns out, although the option to exclude/identify clients via MAC addresses may be present in dhcpv6 servers, it can't be used reliably at all. As the official kea documentation states: “Unfortunately, the DHCPv6 protocol does not provide any completely reliable way to retrieve that information.”