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Edit the /etc/sysctl.d/99-sysctl.conf file:

sudo nano /etc/sysctl.d/99-sysctl.conf 

insert the following at the end:

# Disable IPv6 persistent
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
# optional
# net.ipv6.conf.lo.disable_ipv6 = 1

then: enable and test with iptool:

sudo sysctl -p ; ip a 

Enjoy!