cloud.linode.com says I'm delegated 2600:3c01:e000:0334::/64

/etc/network/interfaces contains this (interfaces.d/* is empty):

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# This is an autoconfigured IPv6 interface
iface eth0 inet6 auto

# The primary network interface
auto eth0
iface eth0 inet static
    address 64.71.152.40/24
    gateway 64.71.152.1
    up   ip addr add 192.168.158.103/17 dev eth0 label eth0:1
    down ip addr del 192.168.158.103/17 dev eth0 label eth0:1

    dns-nameservers 173.230.145.5 173.230.147.5 173.230.155.5 173.255.212.5 173.255.219.5 173.255.241.5 173.255.243.5 173.255.244.5 2600:3c01::5 2600:3c01::6 2600:3c01::7 2600:3c01::8 2600:3c01::9 2600:3c01::b 2600:3c01::c
    dns-search members.linode.com
    dns-options rotate

iface eth0 inet6 static
    address 2600:3c01:e000:334::1/64
    gateway fe80::1


ip -6 addr includes:

3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
    inet6 2600:3c01:e000:334::1/64 scope global
       valid_lft forever preferred_lft forever


$ ip -6 route show
2600:3c01:e000:334::/64 dev eth0  proto kernel  metric 256  pref medium
fe80::/64 dev eth0  proto kernel  metric 256  pref medium
default via fe80::1 dev eth0  metric 1024 onlink  pref medium