Debian 11
## edit /etc/default/grub
sudo vi /etc/default/grub
## Change the 2 lines as follows to add the ipv6.disable=1 into the GRUB CMDLINE
From:
GRUB_CMDLINE_LINUX_DEFAULT="quiet"
GRUB_CMDLINE_LINUX=""
To:
GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 quiet"
GRUB_CMDLINE_LINUX="ipv6.disable=1"
## update grub bootloader by
sudo update-grub
## reboot
sudo reboot
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