In the context of increasing developments of home, building and city automation, the Power Line Communication (PLC) networking medium is called for unpreceeding usage. Our view of the future building networking infrastructure places PLC as the central point. We show in this paper that even if Wireless Sensors Networks (WSN) are good candidates in several cases of the sensor and actuator networking infrastructure, PLC is mandatory in several place of the smart-grid metering and command infrastructure. Also PLC will serve the infrastructure on the sensor/actuator side when the energy requirement cannot be fulfilled by autonomous battery and capacitor based nodes. PLC may provide the numerous bridges necessary to sustain a long lifetime (years) for the WSN infrastructures. This new role of PLC networking will be possible only if the inter-operability between all media and technology is made possible. Thanks to the design of converging IPv6 networking layers, we show that full inter-operability is already possible even in very tiny constrained networking devices. Moreover, low energy PLC, will be able to provide smart grid monitoring without impacting the overall energy balance.