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For optical communication links using wavelength-division multiplexing over a long-haul fiber optic backbone, four-wave mixing (FWM) may lead to significant transmission impairment. Lightwaves traversing through a wavelength-routed optical network (WRON) encounter progressive linear and nonlinear interactions and periodic loss/gain mechanisms through lossy fiber segments and noisy optical amplifiers. In this paper, the FWM power accumulated and received at the end of each lightpath in a WRON is estimated through analytical modeling of lightwaves. While estimating FWM interference for a given lightpath, various feasible lightpath topology scenarios, such as the possibility of any other lightpath joining the given lightpath or departing the given lightpath at any intermediate node, or co-propagating along with the given lightpath till the given lightpath end, are taken into account. The transmission impairments from the accumulated FWM crosstalk along with the amplified spontaneous emission (ASE) noise components from the inline optical amplifiers are considered for evaluating the overall optical signal-to-noise ratios (OSNRs) at the receiving ends of lightpaths. The values of FWM contributions and the OSNR for each lightpath, estimated by using the proposed analytical model, help in setting up lightpaths in a WRON, as one can predict whether the lightpath under consideration could offer a desirable physical-layer performance. From our earlier work [Opt. Switching Networking, vol. 6, pp. 10-19, 2009], though we observe that the count-based assessment of FWM interference (wherein the total number of generated FWM components is estimated from the co-propagating lightpaths) offers a simple method, the power-based evaluation of FWM, as developed in this paper, gives a more precise estimate of the FWM impact of the lightpath in a WRON setting. In view of this, we also carry out FWM-aware and FWM-unaware lightpath topology designs (LTDs) and compare the results of the two - TDs, based on the count-based and the power-based estimates of FWM.