Any device (mac or not) receives its subnet mask from the DHCP service, or by manually configuring the interface. In the case of manual configuration you would need to check with your network administrator to find out the correct setting for the subnet mask.
It does not require a subnet mask.
If this is a default subnet mask, then it would be a class C subnet mask. If you are subnetting a network and this is not the default subnet mask, then it could be either a class A or class B.
The default subnet mask has a standard size. The custom subnet mask allows you to make subnets that are smaller or larger than the default.
There is no technical term called "subnet mass number." It might be a misconstrued term. Subnetting is dividing a network into smaller subnetworks for better organization and security. Mass number is used in chemistry to denote the total number of protons and neutrons in an atom's nucleus.
2 given subnets
255.255.255.0
255.255.255.0
IP Address Additional info: The IP address for a client may not be on the correct subnet; it is possible that the subnet mask is incorrect.
Subnet mask for the above is 255.0.0.0
if the ip address is 192.168.2.1 then the subnet mask should be 255.255.255.0
The default subnet mask would be 255.255.0.0 for a class B address.