Vector Spaces & Bases · Superchallenge problem

Let $N = \begin{pmatrix}0 & 1 & 0 & 0\\0 & 0 & 0 & 0\\0 & 0 & 0 & 1\\0 & 0 & 0 & 0\end{pmatrix}$. What is the dimension of the space of all real $4\times 4$ matrices $X$ with $NX = XN$?