Disabling IPv6 on CentOS 7.5 is easy. This method was tested on a virtual server running in DigitalOcean and persists after reboots. First, let’s check to see that IPv6 is in fact enabled in a default CentOS 7.5 virtual server:

[root@server ~]# ifconfig -a | grep inet6
        inet6 fe80::xxxx:xxxx:xxxx:xxxx  prefixlen 64  scopeid 0x20
        inet6 ::1  prefixlen 128  scopeid 0x10
[root@server ~]# 

Next, let’s issue the following commands to disable IPv6:

[root@server ~]# echo "net.ipv6.conf.all.disable_ipv6 = 1" >> /etc/sysctl.conf
[root@server ~]# echo "net.ipv6.conf.default.disable_ipv6 = 1" >> /etc/sysctl.conf

To make the changes active and verify, let’s reboot the server:

[root@server ~]# shutdown -r now

Finally, we can verify that IPv6 is no longer active on the virtual server:

[root@server ~]# ifconfig -a | grep inet6
[root@server ~]# 

It’s that easy!