Calculus is never boring!!!

Let $f(x)= \dfrac {ax+b}{cx+d}$ , where $a$ , $b$ , $c$ , and $d$ are non-zero reals. If $f (7) =7$ , $f (11) =11$ , and $f (f (x)) = x$ for all $x$ except $-\dfrac dc.$ What is the unique number which is not in the range of $f$ ?