Move from using isc-dhcp to dnsmasq for DHCP services.

[nixfu]

One of the complaints, and wishlist items for DHCP is to keep track of when the 'static' dhcp assignments are given out as well.

To achieve this, I would recommend moving away from isc-dhcpd to using the already installed dnsmasq daemon in zentyal to handle DHCP services as well.   

The popular and proven DD-WRT hardware wifi router firmware uses dnsmasq and it keeps track of both dynamic and static dhcp assignments.