Na verdade, encontrei a solução:
subnet6 2001:470:1f1a:5b3::20/128
Eu tive que procurar na minha ifconifg
Destination Next Hop Flag Met Ref Use If
2001:470:1f1a:5b3::1/128 :: U 1024 1 4 he-ipv6
2001:470:1f1a:5b3::/64 :: Un 256 2 24 he-ipv6
2001:470:1f1a:5b3::/64 :: U 256 1 1 enp2s0
fe80::/64 :: U 256 0 0 enp1s0
fe80::/64 :: Un 256 0 0 he-ipv6
fe80::/64 :: U 256 2 72 enp2s0
::/0 2001:470:1f1a:5b3::1 UG 10 2 11 enp2s0
::/0 2001:470:1f1a:5b3::1 UG 1024 5 68 he-ipv6
::/0 :: !n -1 1109832 lo
::1/128 :: Un 0 3 8351 lo
2001:470:1f1a:5b3::2/128 :: Un 0 2 267 lo
2001:470:1f1a:5b3::20/128 :: Un 0 2 44 lo
fe80::c0a8:4e14/128 :: Un 0 1 0 lo
fe80::9ade:d0ff:fe04:23c3/128 :: Un 0 1 0 lo
fe80::eeaa:a0ff:fe1b:4d84/128 :: Un 0 2 73 lo
ff00::/8 :: U 256 1 5 enp1s0
ff00::/8 :: U 256 0 0 he-ipv6
ff00::/8 :: U 256 1 237 enp2s0
::/0 :: !n -1 1109832 lo