The paper reports an analytical formulation of a block impacting into a metallic net of a rockfall protection fence able to evaluate the net elongation and its braking time. The analytical procedure has been calibrated through a comparison with the experimental results of full-scale impact tests carried out in a vertical-drop test site. The barrier sample is made of three functional modules and is anchored orthogonally to a vertical slope. In the impact test, the block trajectory is vertical and the block impacts into the centre of the middle functional module; no ground contacts occurs after the impact. As a result, this kind of test is particularly suitable for the purposes of model calibration. The metallic net behaviour is assumed to be elastic and the event described by a non homogeneous, second order differential equation with constant coefficients, determined by imposing the initial conditions. The motion equation in the post impact phase is derived by integrating the differential equation: the net maximum elongation and the test block braking time are thus estimated and show a remarkably good agreement with the experimental results.