1. Edit the file - /etc/sysctl.conf

$ sudo gedit /etc/sysctl.conf

And fill in the following lines at the end of that file

# IPv6 disabled
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

2. Save the file and close it

3. Restart sysctl with

$ sudo sysctl -p

4. Check the output of ifconfig again and there should be no ipv6 address

$ ifconfig
eth0      Link encap:Ethernet  HWaddr 08:00:27:5f:28:8b  
          inet addr:192.168.1.3  Bcast:192.168.1.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:1346 errors:0 dropped:0 overruns:0 frame:0
          TX packets:965 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:1501691 (1.5 MB)  TX bytes:104883 (104.8 KB)

If it does not work, then try rebooting the system and check ifconfig again.

---

Disable ipv6 - GRUB method

1. Ipv6 can also be disabled by editing the grub configuration file

$ sudo gedit /etc/default/grub

2. Look for the line containing "GRUB_CMDLINE_LINUX" and edit it as follows

GRUB_CMDLINE_LINUX="ipv6.disable=1"

The same can also be added to the value of the variable named "GRUB_CMDLINE_LINUX_DEFAULT" and either would work. Save the file, close it and regenerate the grub configuration

$ sudo update-grub2

3. Reboot. Now ipv6 should be disabled.

ที่มา : http://www.binarytides.com/disable-ipv6-ubuntu/