Disable ipv6 on CentOS 6

# vi /etc/sysctl.conf

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

Disable in the running system:

# echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6
# echo 1 > /proc/sys/net/ipv6/conf/default/disable_ipv6

or

# sysctl -w net.ipv6.conf.all.disable_ipv6=1

net.ipv6.conf.all.disable_ipv6 = 1 

# sysctl -w net.ipv6.conf.default.disable_ipv6=1

net.ipv6.conf.default.disable_ipv6 = 1