1. A network designing tool for use in designing at least a portion of a communications network, having: an input for receiving an initial network design and a set of traffic requirements; a store for storing at least one network design rule; a rule generator for generating a population of rule sets, each rule set comprising at least one network design rule; a network modifier for applying rule sets generated by the rule generator to create at least one modified network design; performance measurement means for calculating at least one measure of network performance in respect of a modified network design when carrying traffic as defined by an input set of traffic requirements; and an evaluator for comparing modified network designs using respective performance measures calculated by the performance measurement means, for selecting at least one preferred network design satisfying predetermined performance criteria and for identifying a respective rule set used by the network modifier to create said at least one preferred network design; wherein the rule generator is arranged to receive, from the evaluator, the identity of at least one rule set used to create a preferred network design and to generate a new population of rule sets comprising rules derived from said at least one identified rule set, and wherein the network modifier is arranged to apply rule sets from said new population of rule sets to create further modifications of said at least one preferred network design for input to the performance measurement means and for evaluation by the evaluator.