Net Mask
Use the 'ifconfig' command on your adaptor, or just 'ifconfig -a'
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
The default subnet mask would be 255.255.0.0 for a class B address.
if the ip address is 192.168.2.1 then the subnet mask should be 255.255.255.0
Subnet mask for the above is 255.0.0.0
This is a Class B address. So if classful addressing scheme we can say that the default mask of any Class B address is 255.255.0.0. But it may not be always this. There is a concept called VLSM (variable length subnet mask) with which we have the option to give different subnet masks.