The validation of source IP addresses becomes the key technique for devising a trustworthy network. However, inter-domain IP spoofing preventions based on source-destination labels and end-hosts IP authentications based on source labels both adopt end to end mode to solve the problem, which ignores the flooding of spoofing packets on middle networks. To address this problem, an enhancing mechanism for the inter-domain IP spoofing prevention service, ESP (enhanced spoofing prevention), is proposed. Via integrating path labels into source labels, ESP reduces the collision of source labels at destination networks and enables filtering IP spoofing packets toward other nodes in middle networks, thus prevents flooding attacks in advance and extends the protected domain of the spoofing prevention. Based on BGP (border gateway protocol) update ESP develops the validation of prefix security to restrict the scope of the propagation of labels, thus decreases the cost of computing and storing of labels. The abilities of IP spoofing prevention and filtering spoofing packets in advance are demonstrated in the topology, which is constructed based on RIB (routing information base) provided by Routeview.