It depends, you could use 255.255.254.0 which will allow you 512 machines, however you are limiting yourself to 512 as a maximum. You can calculate any subnet mask you like at www.subnetmask.info
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.
255.255.255.0
2 given subnets
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
To create a subnet ID, you start with the network's base IP address and subnet mask. The subnet mask determines how many bits are allocated for the network and how many for the hosts. By applying the subnet mask to the base IP address, you can identify the subnet ID, which represents the network portion of the address. For example, if your base IP is 192.168.1.0 with a subnet mask of 255.255.255.0, the subnet ID is 192.168.1.0.
By changing subnet mask..from 255.255.224.0 to 255.255.240.0 Number of subnetworks increase and the number of host in each subnetwork decreases