The amount of electricity consumed by devices connected to the Internet in the U.S. has rapidly increased and now amounts to over 2% of total electricity used, which is about 74 TWh/yr costing over $6 billion annually. This energy use can be categorized as direct and induced. Much of this energy powers idle links, switches, and network-connected hosts and is thus wasted.This dissertation contains the first-ever investigation into the energy efficiency of Ethernet networks. A method for matching Ethernet link data rate with link utilization, called Adaptive Link Rate (ALR), is designed and evaluated. ALR consists of a mechanism to change the link data rate and a policy to determine when to change the data rate. The focus of this dissertation is on the analysis and simulation evaluation of two ALR policies. The simplest ALR policy uses output buffer thresholds to determine when to change data rate. This policy is modeled using a Markov chain. A specific challenge was modeling a state-dependent service rate queue with rate transition only at service completion. This policy was shown to be unstable in some cases, and an improved policy based on explicit utilization measurement was investigated. This more complex policy was evaluated using simulation. A synthetic traffic generator was developed to create realistic synthetic network traffic traces for the simulation evaluation. Finally, an improved method for detecting long idle periods using quantile estimation was investigated.Characterization of network traffic showed that proxying by a low power device for a high power device is feasible. A prototype proxy for a Web server was developed and studied. To maintain TCP connections during sleep time periods of a host, a new split TCP connection method was designed. The split connection method was prototyped and shown to be invisible to a telnet session.This research has contributed to the formation of an IEEE 802.3 Energy Efficient Ethernet study group. It is thus very likely that ALR will become a standard and will achieve industry implementation and widespread deployment. This will result in energy savings of hundreds of millions of dollars per year in the U.S. alone.