I know how the subnet works, and what it does need. Unfortunately this ip address I have is only provided at isp dhcp (adsl modem) level, which sets it with /32 mask, and designated gateway in routing table entry.
For me it looks that gw is from another address pool, unless the isp is using /16 network for its dhcp leases.
I'd rather not put /16 netmask in my configuration, and if You can not see any other way, I'll write some script to set interface address and routing on start, and leave the interface without configuration in zentyal, though loosing all gui-friendly firewall/proxy/etc.. features on