Which of these is not a valid subnet mask?
Answer
255.255.253.0
A mask is a run of 1s followed by a run of 0s, with no mixing. 253 is 11111101 in binary — a 0 with a 1 after it — so it cannot be a mask, and an interface configured with it will be rejected or will behave unpredictably. The other three are clean: 128 is 10000000, 192 is 11000000, and 240 is 11110000, each a valid boundary. This is the quickest reason to be able to convert an octet in your head, since a bad mask is invisible in decimal and obvious in binary.