Since 157.54.4.201 falls in the class B address range the default subnet mask would be:
255.255.0.0
128 - 191 is a class B address.
The default subnet mask would be 255.255.0.0 for a class B address.
If your address is not subnetted, then the default subnet mask would be used. For this class address, it would be 255.0.0.0
There isn't just a single subnet mask that could be used for that address. The default subnet mask would be 255.0.0.0, but there could be others if the network is subnetted.
255.255.255.0 is most common.
The default subnet would be 255.0.0.0 (class A)
That would be a class E network, E networks do not have defines subnet masks or networks. Just a range (240.0.0.0 - 255.255.255.255).
Since this is a class B with the default subnet mask the network would be: 172.32.0.0
Since this is a class C address the subnet mask for the default would be 255.255.255.0. Other values could be:255.255.255.128 - 255.255.255.252
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.
172.16.0.0. The first ip address within this subnet is 172.16.0.1
The subnet address for this ISP could be almost anything, depending on how the ISP breaks up a larger network pool into a smaller one. The default subnet would be 255.0.0.0, but that is not very likely with an ISP. You can check by using the 'ipconfig' command in Windows to see the subnet mask, or 'ifconfig' if using Linux.
It depends on whether you are subnetting or not. If not, the default subnet mask would be 255.255.255.0