Disable IPv6 on Debian Jessie

From https://wiki.debian.org/DebianIPv6#How_to_turn_off_IPv6
Append ipv6.disable=1 to the GRUB_CMDLINE_LINUX variable in /etc/default/grub.
Run update-grub and reboot.
or better,
edit /etc/sysctl.conf and add those parameters to kernel. Also be sure to add extra lines for other network interfaces you want to disable IPv6.

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
net.ipv6.conf.eth0.disable_ipv6 = 1

After editing sysctl.conf, you should run sysctl -p to activate changes or reboot system.