Disable IPv6 in Debian system

Step 1.
# vi /etc/modprobe.d/aliases
or
# vi /etc/modprobe.d/aliases.conf

Step 2.
find

alias net-pf-10 ipv6

Replace with

alias net-pf-10 off
alias ipv6 off

Ref:http://www.cyberciti.biz/tips/linux-how-to-disable-the-ipv6-protocol.html