Binary Octet

Build a number from its 8 bits, then choose how many bits are fixed. The free bits decide the block size, and zeroing or filling them gives the block's first and last value. This is the whole trick behind subnet math.

Decimal= 128 + 64 + 8

Fixed bits in this octet4 fixed / 4 free

Block size (2 ^ free bits)16

Block start (free bits = 0)1921100|0000

Block end (free bits = 1)2071100|1111

Quick set

The fixed bits are the network part. Block starts are always multiples of the block size (16), so 200 lives in the block 192–207.

As a subnet mask octet, 4 fixed bits is 256 − 16 = 240. In the 4th octet that's a /28, in the 3rd a /20.