What a Mask Does

The slash tells humans where the network part ends. Machines use a subnet mask instead: 32 bits where every network bit is 1 and every host bit is 0.

For /24 the mask is 24 ones followed by 8 zeros. Written as octets, that is 255.255.255.0: three full bytes of ones (255 each) and one byte of zeros.

The ones always come first, with no gaps. 255.0.255.0 is not a legal mask; once the zeros start, they run to the end.