The device's administrative IP subnet mask. The device
may automatically discover a value for this object using
the BootStrap protocol (BOOTP). The object value is also
duplicated in the MIB-II ipAddrTable.
This object is writable only if the value of
ipConfigMethod is set to manual(1).
A write to this value will take effect immediately.
The default value is 0.0.0.0, or no subnet mask.