The equivalent resistance of the circuit shown below between points...

The resistance in $ab$ is parallel to all other resistances. Since all the resistances other than ${R}_{b}$ is $4Ω$ the potential is zero$\left({V}_{x} = {V}_{y}\right)$ as the upper branch will become a case of balanced wheatstone bridge between ${V}_{x} \text{ and } {V}_{y}$. So, the simplified circuit looks like the one below.

Hence, the equivalent resistance is

$\dfrac{1}{{R}_{eq}} = \dfrac{1}{8} + \dfrac{1}{8} + \dfrac{1}{16} = \dfrac{5}{16} ⇒ {R}_{eq} = \dfrac{16}{5}Ω = 3 . 2Ω$