From a comment I came across on YouTube:

If you don’t have that chart you can still figure? out the mask. An ip(V4) address is 32 bit long.(4 * 8) Four digits, each an octet. The mask is 28 bit long, all you have to do is, to subtract 28 from 32. 32 – 28 = 4. So now you know the first 28 bits are set to 1, and last 4 bits are set to 0. Last octet is going to be look like this 11110000. Convert this number back to decimal = 240. So your mask is going to be In binary the whole thing: 11111111.11111111.11111111.111­10000.
– powernodes