1 1 1 1 1 1 1 1
128 * 64 * 32 * 16 * 8 * 4 * 2 * 1
If you simply memorize the multiples above, you can easily convert a binary to decimal and vice versa. You don't need no stinkin' calculator!
Use the following chart below to use for your shortcut formula to creating a subnetted mask
128 1 0 0 0 0 0 0 0
192 1 1 0 0 0 0 0 0
224 1 1 1 0 0 0 0 0
240 1 1 1 1 0 0 0 0
248 1 1 1 1 1 0 0 0
252 1 1 1 1 1 1 0 0
254 1 1 1 1 1 1 1 0
255 1 1 1 1 1 1 1 1