The integration of inverter-interfaced distributed generation (IIDG) alters the fault current characteristics of distribution networks, which can lead to reduced sensitivity and coordination failure of three-stage overcurrent protection under fixed settings. To evaluate the maximum hosting capacity of IIDG without modifying protection settings, a capacity assessment model is formulated incorporating constraints on protection performance, voltage deviation, penetration level, and individual unit capacity. Based on the principles of overcurrent protection coordination, the concept of protection range is introduced as a unified index to characterize both sensitivity and selectivity, and a protection constraint model under fixed settings is derived. To address the need for iterative short-circuit current calculations, an explicit modeling approach for fault currents is proposed and embedded into the optimization framework. By aggregating IIDGs based on protection impact similarity, the original mixed-integer nonlinear problem is converted into a mixed-integer linear programming model through piecewise linearization. MATLAB/Simulink simulations verify that when the IIDG capacity remains within the derived boundary, protection operates correctly and node voltages stay within permissible limits, verifying the effectiveness of the proposed approach.