Disable IPv6 using sysctl settings (no reboot required)

 1. Append below lines in /etc/sysctl.conf:

net.ipv6.conf.all.disable_ipv6 = 1

net.ipv6.conf.default.disable_ipv6 = 1

NOTE : To disable IPv6 on a single interface add below lines to /etc/sysctl.conf :

net.ipv6.conf.[interface].disable_ipv6 = 1 ### put interface name here [interface]

net.ipv6.conf.default.disable_ipv6 = 1

2. To make the settings affective, execute :

# sysctl -p

NOTE : make sure the file /etc/ssh/sshd_config contains the line AddressFamily inet to avoid breaking SSH Xforwarding if you are using the sysctl method

3. Add the AddressFamily line to sshd_config :

# vi /etc/ssh/sshd_config

....

AddressFamily inet

....

Restart sshd for changes to get get effect :

# systemctl restart sshd

https://www.thegeekdiary.com/centos-rhel-7-how-to-disable-ipv6/