I have trouble understanding the meaning of $d$-spacing. $d$-spacing is supposed to give the interplanar distance. For a cubic lattice
$$d_{hk \ell}= \frac {a} { \sqrt{h^2 + k^2 + \ell ^2} } $$
What i know is 100 plane and 200 plane are parallel, when i calculate the $d$-spacing for 100 plane and 200 plane i get two different answers; as the formula itself involves $hkl$. What is $d$-spacing then? Distance between parallel set of planes?