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Optimization of IP over dense wavelength division multiplexing (DWDM) networks is studied under the constraints of energy and cost minimization. Realistic IP traffic mapped to the optical DWDM layer together with leased lines form the basic traffic load, which is expanded by factors of 33 and 100 for a 10-year projection. The DWDM core network (USA 66) is analyzed for mixed-line-rate (MLR) and reach/bandwidth-adaptive scenarios; the latter is realized by optical orthogonal frequency division multiplexing. Both scenarios are studied for flat and hierarchical network approaches and for different channel grid configurations. Network optimization due to capital expenditure and power consumption yield no significant differences for the IP layer regarding different scenarios (flat and hierarchical) or network approaches (MLR and adaptive network). In contrast, the wavelength division multiplexing layer shows benefits for the hierarchical network regarding cost and power and for the adaptive approach regarding the total number of transponders and fibers used. We show that elastic and MLR networks can achieve comparable performance levels using a 12.5 GHz grid. This article is a more detailed version of Klekamp et al., OFC, 2012, OTh3B.1, and extends previous studies in a German network [J. Lightwave Technol., vol. 30, p. 215, 2012], now comparing the techniques in a US network under different conditions regarding distances and topology.