I have an ethernet connection (to a SiliconDust box) that requires a netmask 255.255.0.0. So, no problem

% ifconfig eno1 192.168.10.6  netmask 255.255.0.0
% ifconfig eno1
eno1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.10.6  netmask 255.255.0.0  broadcast 192.168.255.255

some time later ...

% ifconfig eno1
eno1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.10.6  netmask 255.255.255.0  broadcast 192.168.10.255

Any ideas on why the change does not stick?

Thanks