If you have access to the Red Hat knowledge base, there's an article here:
https://access.redhat.com/knowledge/solutions/8709
And in case you don't have access, here's a copy/paste:
Red Hat Enterprise Linux 4, 5, and 6 enable Internet Protocol Version 6 (IPv6) by default. However, in certain situations, some users may find it desirable to disable IPv6 support or to re-enable it after it has been disabled.
For Red Hat Enterprise Linux 6, the tech brief Preparing for IPv6 Networking in Red Hat Enterprise Linux provides a broad introduction to IPv6 networking.
Disabling IPv6 support in Red Hat Enterprise Linux 6
Create a file /etc/modprobe.d/ipv6.conf with the following contents:
options ipv6 disable=1
For completeness, it is a good idea to configure the ip6tables service not to start at boot by issuing the following command:
# chkconfig ip6tables off
Reboot the system to disable IPv6 support.
Note: There is a special case where this might not work, please see The "ipv6 disable=1" option does not seem to work on Red Hat Enterprise Linux 6 system.
Re-enabling IPv6 support in Red Hat Enterprise Linux 6
Review the files under /etc/modprobe.d/ and remove (or comment out) any of the following lines:
options ipv6 disable=1
install ipv6 /bin/true
blacklist ipv6
Configure the ip6tables service to start at boot by issuing the following command:
# chkconfig ip6tables on
Reboot the system to activate IPv6 support.
Disabling IPv6 support in Red Hat Enterprise Linux 5
Remove the following line (if present) from the /etc/modprobe.conf file:
alias net-pf-10 ipv6
Add the following line to the /etc/modprobe.conf file:
alias net-pf-10 off
In versions of Red Hat Enterprise Linux before 5.4, add the following line to the /etc/modprobe.conf file:
alias ipv6 off
In Red Hat Enterprise Linux 5.4 and later, add the following line to the /etc/modprobe.conf file:
options ipv6 disable=1
To prevent errors during the network initscript start routine, change the NETWORKING_IPV6 parameter in the /etc/sysconfig/network file to the following:
NETWORKING_IPV6=no
For completeness, it is a good idea to configure the ip6tables service not to start at boot by issuing the following command:
# chkconfig ip6tables off
Reboot the system to disable IPv6 support.
Re-enabling IPv6 support in Red Hat Enterprise Linux 5
Remove the following lines (if present) from the /etc/modprobe.conf file:
alias net-pf-10 off
alias ipv6 off
options ipv6 disable=1
Add the following line to the /etc/modprobe.conf file (if not present already):
alias net-pf-10 ipv6
Change the NETWORKING_IPV6 parameter in the /etc/sysconfig/network file to the following:
NETWORKING_IPV6=yes
For completeness, it is a good idea to configure the ip6tables service to start at boot by issuing the following command:
# chkconfig ip6tables on
Reboot the system to re-enable IPv6 support.
Disabling IPv6 support in Red Hat Enterprise Linux 4
Remove the following line (if present) from the /etc/modprobe.conf file:
alias net-pf-10 ipv6
Add the following line to the /etc/modprobe.conf file:
alias net-pf-10 off
Reboot the system to disable IPv6 support.
Re-enabling IPv6 support in Red Hat Enterprise Linux 4
Remove the following line from the /etc/modprobe.conf file:
alias net-pf-10 off
Add the following line to the /etc/modprobe.conf file:
alias net-pf-10 ipv6
Reboot the system to re-enable IPv6 support.