The subnet mask would be 255.255.255.0
No, 255.247.0.0 is not a valid subnet mask. A valid subnet mask must have contiguous bits set to 1 followed by contiguous bits set to 0, which means that after the 1s, all subsequent bits should be 0. The binary representation of 255.247.0.0 does not adhere to this rule, as it has non-contiguous 1s and 0s.
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.
Subnet mask.
2 given subnets
255.255.255.0
255.255.255.0
If you represent it in Hexadecimal, sure. That's very uncommon in IPv4, but just fine for IPv6.
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