The water distribution network design problem is quite complicated due to the non-lineal relation between the flow and the head loss (load) and to the presence of discrete variables, such as the diameters of the pipes, which can only have discrete values according to it's availability in the market. Additionally the cost function of the pipes has a non-lineal relation with the diameters. However, CIACUA (Water Distribution and Sewer Systems Research Center of Universidad de los Andes) has worked on this problem, considering it as a problem of combinatorial optimization where the variables of decision are the diameters of each one of the pipes of the network. Based on this works, the optimal cost design problem is considered solve. This optimal cost design is obtained based on the Maximum Hourly Flow (MHF) in the network, for the design period (established by local or national regulations). Now, based on the Resilience Index concept (the relation between the power per weight unit dissipated by the WDN and the power per weight unit available to be dissipated), developed by Todini, this optimal cost design would represent a network with a low reliability. The main objective of this research is to demonstrated, using the hydraulic model of a real network, that the optimal cost design corresponds to a network with a good reliability for its initial operation time. The network reliability will decrease with time, until reach its lower level at design period. The WDN operators must assess periodically the network resilience, looking for keep a good reliability of the service.