Basic rule for subnet mask

The default subnet mask that you add for ICD files that don't have a ConnectedAP should follow this logic:

Class A—Any IP address whose first octet is between 1 and 126 is a Class A address. The default Class A subnet mask is 255.0.0.0

Class B—Any address whose first octet is in the range 128 to 191 is a Class B address. The default Class B subnet mask is 255.255.0.0

Class C—The first octet range of 192 to 223 is a Class C address. The default Class C subnet mask is 255.255.255.0