A simple way of finding out your computer's external subnet mask in Windows is to go to Start and select Run. Type ipconfig into the box, press the Enter key on the keyboard and a dialogue box will pop up with the required information.
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.
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.
The correct term is subnet mask instead of subnet mass. A subnet mask separates an IP address into the host address and the network address. It is a 32-bit number that can mask an IP address.
If the other PC is on the same LAN segment then it will have the same subnet mask as yours. A subnet mask is the same for all subnets within a network so that will be the same as well. For other networks it isn't important to know what their subnet is. You could always ask the other user to find out that information if you needed to know.
Google it.
2 given subnets
255.255.255.0
255.255.255.0
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