/etc/network/interfaces

# The loopback network interface
auto lo
iface lo inet loopback
 
# The primary network interface
allow-hotplug eth0
iface eth0 inet static
        address 192.168.21.2/24
        gateway 192.168.21.1
iface eth0 inet6 static
        address fd18:76a0:3427:201::2/64
iface eth0 inet6 auto

/etc/network/interfaces

iface eth0 inet6 auto #SLAAC
    pre-up /sbin/ip token set ::48 dev eth0 #configure static token with changing prefix
 
iface eth1 inet6 dhcp #DHCPv6

¹⁾

/etc/sysctl.conf

net.ipv6.conf.default.addr_gen_mode = 0
net.ipv6.conf.eth0.addr_gen_mode = 0

ifupdown sends dhclient -i Flag (use DHCPv6 DUID for v4) by default now. To revert backt to MAC-Address set following item in dhclient.conf:²⁾

/etc/dhcp/dhclient.conf

send dhcp-client-identifier = hardware;

or this option in /etc/network/interface:³⁾

/etc/network/interfaces

iface eth0 inet dhcp
    client no

/etc/sysctl.conf

net.ipv6.conf.eth0.disable_ipv6 = 1 #replace eth0 with desired interface or all

/etc/netplan/50-cloud-init.yaml

# This file is generated from information provided by
# the datasource.  Changes to it will not persist across an instance.
# To disable cloud-init's network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
 
 
network:
    ethernets:
        eth0:
            dhcp4: no
            addresses:
            - 192.168.25.40/24
            - 2001:6b0:1234:1470::40/64
            gateway4: 192.168.25.1
            gateway6: 2001:6b0:1234:1470::1
    version: 2

sudo netplan apply

not persistent

sudo ip token set ::27 dev eth0

persistent

add token addr and fe80 with that number

/etc/network/interfaces

iface eth0 inet6 auto
        pre-up /sbin/ip token set ::40 dev eth0
        up /sbin/ip -6 addr add fe80::40/64 dev eth0

sudo ip addr add 2003:59:6034::26:4/64 dev eth1
sudo ip route add ::/0 via 2003:59:6034::1 dev eth1

https://gist.github.com/xchewtoyx/f90e4696e9726711673b81d8aff8acff#file-gistfile1-txt