How to disable V6 DHCP ?

[Pierre Couderc]

I have tried to comment INTERFACESv6 in /etc/default/isc-dhcp-server
but systemd  tries to start dhcp server ipV6, fails as it is not 
initiliased and remains in failed status, even if DSCP ipv4 is welle 
started.
But it refuses to start at next power on.
So how should I start DHCP v4 only... ?

Thanks in advance
PC