To devise effective network engineering strategies and to assess the quality of upstream providers, network operators would greatly benefit from the knowledge of which Internet paths might be traversed by the traffic flows entering their networks in the case of network faults or when traffic engineering measures are used. However, current methodologies do not provide this information. This paper presents methodologies to discover alternate paths that might be selected in the presence of network faults or different routing policies and to deduce the routing policies of other operators. The techniques are validated through extensive experimentation on the Internet. -