In the context of increasing developments of home, building and city automation, Power Line Communication (PLC) networking is called for unprecedented usage, especially for energy efficiency improvement. Our view of the future building networking infrastructure places PLC as the central point. We point that while Wireless Sensor Network (WSN) is necessary in the sensor and actuator networking infrastructure, PLC is mandatory for the smart-grid metering and command infrastructure. PLC will also serve the sensor/actuator infrastructure when the energy requirement of the probing system itself cannot be fulfilled by autonomous battery and harvesting based nodes. PLC may also provide the numerous bridges necessary to sustain a long lifetime (years) with the battery based WSN part of the infrastructure. This new role of PLC networking will be possible only if the interoperability between all media and technology is made possible. Thanks to the converging design of Internet Protocol version 6 (IPv6) networking layers, we show that such a full inter-interoperability is already possible even in very tiny constrained networking devices. Moreover, the low power PLC technology used in our experiments will be able to provide this smart grid monitoring without impacting noticeably the overall energy balance of the monitored system.