  While the front of a fluid shock is a few mean-free-paths thick, the front of
a collisionless shock can be orders of magnitude thinner. By bridging between a
collisional and a collisionless formalism, we assess the transition between
these two regimes. We consider non-relativistic, un-magnetized, planar shocks
in electron/ion plasmas. In addition, our treatment of the collisionless regime
is restricted to high Mach number electrostatic shocks. We find that the
transition can be parameterized by the upstream plasma parameter $\Lambda$
which measures the coupling of the upstream medium. For $\Lambda \lesssim
1.12$, the upstream is collisional, i.e. strongly coupled, and the strong shock
front is about $\mathcal{M}_1 \lambda_{\mathrm{mfp},1}$ thick, where
$\lambda_{\mathrm{mfp},1}$ and $\mathcal{M}_1 $ are the upstream mean-free-path
and Mach number respectively. A transition occurs for $\Lambda \sim 1.12$
beyond which the front is $\sim \mathcal{M}_1\lambda_{\mathrm{mfp},1}\ln
\Lambda/\Lambda$ thick for $\Lambda\gtrsim 1.12$. Considering $\Lambda$ can
reach billions in astrophysical settings, this allows to understand how the
front of a collisionless shock can be orders of magnitude smaller than the
mean-free-path, and how physics transitions continuously between these 2
extremes.
