The fast development of the Internet also results in sharp rise in network energy consumption.The energy efficiency of present network devices is very low, which is an obstacle in realizing energy-proportional computing.However, while networks are designed for peak hour load, actual network load can be quite low in a large fraction of time, leaving great opportunity for energy saving.After an introduction of the energy consumption model of network devices, theories and technologies for optimizing network energy consumption are studied from two aspects.First, when the total amount of network traffic cannot be changed, it is possible to add power and performance states to individual network devices and optimize local control policies to realize energy proportionality;or, without sophisticated power management capabilities, it is also possible for a network as an ensemble to realize energy proportionality through coordination and traffic engineering.Second, it is possible to reduce or alleviate the traffic by providing cache capability to the network, which in turn reduces or alleviates the growth of network transmission energy.Intelligent cache deployment, content cache and request routing policies can further optimize network energy consumption.Based on the above discussion, a thorough comparison between different technologies is provided and potential future research directions are analyzed.