====== ipv6 ======
Thank you all for the suggestions provided, and indeed it helps to disable IPv6 completely and remove the module from memory as follows:

Contents of the /etc/modprobe.d/ipv6.conf
<code>
alias net-pf-10 off
alias ipv6 off
options ipv6 disable_ipv6=1
blacklist ipv6
</code>

After a reboot, firing the command "lsmod" shall show no line entry denoting to "ipv6.", and the same shall also be for the command "ifconfig".

Optionally and if required, add in /boot/cmdtline.txt the parameter:

  ipv6.disable=1

It works also for the latest April 2017 Raspbian image with kernel version 4.9.22-v7+.