1. A computer-based method for determining a probability that no path exists from a specified starting node to a specified target node within a network of nodes and directional links between pairs of nodes which form paths of a reliability graph, said method performed using a computer coupled to a database that includes data relating to the nodes and the directional links, said method comprising: selecting a set of paths, from the set of all paths, between the starting node and the target node that have been determined to have a reliability above a selected threshold; calculating a reliability of the union of the selected path sets; setting an upper bound for the unreliability of the set of all paths, based on the calculated reliability; selecting a set of minimal cutsets from all such cutsets that lie between the starting node and the target node, a minimal cutset being a cutset where a subset of the directional links in the subset is not a cutset; calculating the probability of the union of the minimal cutsets; and setting a lower bound for the unreliability of the set of all cutsets.