Abstract: A method and system for determining the best data path in a communication network is presented. A Least Cost Routing path between end users in a multi-network environment, including both public and private networks, is determined based upon a longest prefix match. At network activation time, the numbers or prefixes of numbers of end users attached to the public network (304, 305) are stored in entry node data bases A and C of the private network (303). Assuming an end user terminal U1 calls a target end user U3, an entry node B first checks for a longest prefix match between the called number and numbers already stored into its memory. If this is the first time U3 is called via node B, no match will occur and node B will then broadcast a Query to all nodes in the private network (i.e. Nodes A and C). Only those nodes providing possible access to U3 answer this Query.