Full-scale IT introduction in the near future is expected to increase information flows vastly and this information explosion will also increase the number of IT devices in use, positioning the energy consumption of IT devices themselves as a key issue. Most of conventional measures to treat energy consumption handle servers and network devices separately. This paper first identifies that the network should cooperate with end systems to reduce the total energy consumption, and proposes an integrated method to reduce the total energy consumption by both network and end systems. Then, this paper proposes a simple method of estimating the volume of energy consumption by all network devices and assigning it to an individual user, based on the volume of traffic measured and the information about packet routing paths in the network.