Ubuntu Isc-Dhcp 4.2.4 and disable IPv6 !

[CpServiceSPb .]

I have isc-dhcp-server installed at ubuntu 14.04 x64.
Its version is 4.2.4

I have fully disabled IPv6 at OS.
Also I have "-4" option for dhcp server startup.

But there is one IPv6 listened address by netstat -anupt.
Port is dyynamic.


What is this dynamically defined port intended for ?

And how is to fully disable IPv6 listening ?
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