The growing concern of global climate change calls for a revolutionary change on the way of utilizing the energy. In terms of the ICT industry, energy efficiency needs to be considered as an important metric to evaluate a communication system, besides the conventional metrics such as quality of service, bandwidth efficiency, etc. Research should be done to raise the energy efficiency of communication networks step by step. As a starting point, this paper provides an overview of communication networks from an energy-efficiency point of view. A series of energy consumption models are summarized as well as the general approaches to develop these models. Simulation results are presented to demonstrate the great influence of protocol design on the overall energy efficiency.