RFID tags will really become smart objects of the Internet of Things (IoT) if they will be able to exploit the passive nature of the communication also for exchanging data with any other networked devices in the Internet. To this aim, an adaptation layer, named 6lo-RFID, has been specifically designed to run IPv6 over the RFID link technology. In this paper we draw the design principles for the development of a fully passive, IPv6-compliant, and energy-efficient 6lo-RFID tag platform. In particular, we discuss the block's design of an RF energy harvester and evaluate the simulated performance of a fully-passive 6lo-RFID tag when GET/PUT CoAP methods are used to execute operations on the tag memory.