Address plays an important role in WSN (wireless sensor networks), which is used to identify sensor node and enable communication protocols. Due to the numerous sensor nodes and network dynamics in WSN, it is difficult even impossible to configure address for each node manually. With its unique characteristics, addressing protocols for TCP/IP and ad hoc networks are not suitable for WSN. So addressing auto-configuration protocol for WSN has to be devised. Firstly, the need for addressing protocol in WSN is analyzed. The research issues and challenges of designing addressing protocol are also summarized. Then, the present representative addressing protocols are classified, introduced in detail and compared in characteristics and performance. Finally, the open issues and future research directions are pointed out.