Every device that speaks IPv4 holds a coordinate in a space of
exactly 4,294,967,296 positions — two to the thirty-second power. Subnetting is the
practice of carving that space into smaller, meaningful neighbourhoods. To carve it well, first you
must read it.
Each of the four numbers in 192.168.42.17 is an octet: eight binary
digits, with values from 0 to 255. Concatenated, they form the full
thirty-two-bit address. The diagram opposite shows every bit, drawn at full scale.
Reading binary is mostly a parlor trick: each position in an octet represents a power of two —
128 64 32 16 8 4 2 1 from left to right. Turn a bit on, add its value. Add them all,
and you have the decimal value of the octet. Subnetting will lean on this constantly.
The address alone is not enough. 192.168.42.17 says nothing about which devices share
a neighbourhood with it. For that, we need a mask — the subject of Article II.