We present a detailed study about the role of the short-cuts of a network in promoting the emergence of cooperation in a population of agents playing the Prisoner's Dilemma Game (PDG). We introduce a model which allows to tune the topology of the system from the one-dimensional euclidean lattice (a ring) to the complete graph just changing the value of one parameter (the probability p to add a link between two nodes not already connected in the eucledean configuration). We show that there is a region of values of p in which cooperation is largely enhanced, whilst for smaller values of p only a few cooperators are present in the final state, and for p 1cooperation is totally suppressed. We present analytical arguments that provide a very plausible interpretation of the simulation results. Our work makes it clear how short-cuts can be decisive in promoting (or suppressing) cooperation in the absence of other mechanisms such as clustering. Implications for other dynamics are also dr...