Setting netmask permanently

[Geoffrey Leach]

Device vendor insists on a netmask 255.255.0.0, so
% ifconfig eno1 169.254.10.6  netmask 255.255.0.0
which changes netmask, but it soon reverts to the original
value 255.255.255.255

ifdown/ifup does not change the situation

Any ideas?