Network device without IPv6? Nonsense, I tell you. Look which year it is. If your ISP is retarded - keep the pace with

IPv6 Tunnel Broker

ifupdown and dhclient for ipv6

If you're using dhcp for both ipv4 and ipv6 on ubuntu with wifi/wpa_supplicant you might have faced with weird issue with ifdown trying to release v6 (or v4, depends on ordering) lease and it finally hangs - since previous address family already brought interface down.