in Ops

Disable SSH connections over IPv6

If a network interface offers IPv4 as well as IPv6, by default Ubuntu’s SSH server listens on both IP versions. In order to restrict access to protocol version 4 you have to comment in the line
#ListenAddress in your /etc/ssh/sshd_config.

#ListenAddress ::

Restart SSH service for the changes to take effect.

sudo service sshd restart