tried both with and without shared network.

ddns-updates off;
ddns-update-style interim;
option domain-name "causeway.stirlingprop.com";
option ntp-servers gw-1.causeway.stirlingprop.com;
option time-offset -21600;
next-server pbx-1.causeway.stirlingprop.com;
option tftp-server-name "pbx-1.causeway.stirlingprop.com";
default-lease-time 21600;
max-lease-time 43200;
one-lease-per-client on;
	option domain-name-servers 172.16.12.7, 172.16.12.8;


shared-network CAUSEWAY {

subnet 172.16.12.0 netmask 255.255.254.0 {

	authoritative;

	option routers 172.16.12.1;
	option subnet-mask 255.255.254.0;
	option broadcast-address 172.16.13.255;

	range dynamic-bootp 172.16.12.64 172.16.12.127;
	range dynamic-bootp 172.16.12.130 172.16.12.254;
	range dynamic-bootp 172.16.13.2 172.16.13.254;
	host bw-laser-1.causeway {
		hardware ethernet 00:01:e6:5f:30:0c;
		fixed-address 172.16.12.32;
	}
	host bw-laser-2.causeway {
		hardware ethernet 00:10:83:0a:be:12;
		fixed-address 172.16.12.33;
	}
	host bw-laser-3.causeway {
		hardware ethernet 08:00:09:c0:70:78;
		fixed-address 172.16.12.34;
	}
	host bw-laser-4.causeway {
		hardware ethernet 00:30:c1:31:59:0a;
		fixed-address 172.16.12.37;
	}
        host co-laser-1.causeway {
		hardware ethernet 00:c0:85:30:6f:05;
		fixed-address 172.16.12.35;
	}
	host switch-1.causeway {
		hardware ethernet 00:10:db:72:87:aF;
		fixed-address 172.16.12.1;
	}
	host power-1.causeway {
		hardware ethernet 00:10:db:72:87:aF;
		fixed-address 172.16.12.3;
	}
	host btel.causeway {
		hardware ethernet 00:04:76:48:8C:2D;
		fixed-address 172.16.12.6;
	}
	host btel-ata.causeway {
		hardware ethernet 00:0e:08:ea:c9:d8;
		fixed-address 172.16.12.5;
	}
}

subnet 172.16.18.0 netmask 255.255.255.0 {

	authoritative;

	option routers 172.16.18.1;
	option subnet-mask 255.255.255.0;
	option broadcast-address 172.16.18.255;

	range dynamic-bootp 172.16.18.64 172.16.18.254;
}

subnet 172.16.19.0 netmask 255.255.255.0 {

	authoritative;

	option routers 172.16.19.1;
	option subnet-mask 255.255.255.0;
	option broadcast-address 172.16.19.255;

	range dynamic-bootp 172.16.19.64 172.16.19.254;
}

}