Next-generation communication protocol IPv6 !! That said, I don't think it's widely used, It tends to interfere with other communications, so disable it.
Confirmed on CentOS 7.4, Ubuntu 16.4
ip a
If it is enabled, inet6 will appear as below. This is ↓.
inet6 fe80::4dd:7eff:fe3d:383f/64 scope link
To disable it, add an item to /etc/sysctl.conf.
/etc/sysctl.conf
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
sysctl -p
After checking, inet6 has disappeared. Is it possible to completely abolish IPv4? https://ja.wikipedia.org/wiki/IP%E3%82%A2%E3%83%89%E3%83%AC%E3%82%B9%E6%9E%AF%E6%B8%87%E5%95%8F%E9%A1%8C