Much work has been done on building content-based publish/subscribe systems over structured P2P networks, so that the two technologies can be combined together to better support large-scale and highly dynamic systems. However, existing content-based routing protocols can only provide weak reliability guarantees over P2P networks. Based on the routing protocols of structured P2P networks, a new type of content-based routing protocol for pub/sub systems is designed, which is called Identifier Range Based Routing (IRBR) protocol. The IRBR protocol guarantees that the subscribing nodes always receive the interested events exactly once as long as the message from publishing nodes to subscribing nodes is arrivable in the P2P network. At the same time, it can also disseminate an event to all interested subscribers with less network traffic. A prototype pub/sub system has been developed on Pastry, and the experimental results demonstrate the fault-tolerance and routing efficiency of the protocol.