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An accurate analysis of the four-wave mixing (FWM) impact on dense wavelength-division multiplexing optical systems is carried out for different types of fibers. Particular channel allocations on the ITU grid are studied to reduce the inband FWM crosstalk and guarantee high performances in different types of optical fiber. These schemes, with respect to the other known channel allocations, allow one to find an optimum tradeoff between the required bandwidth expansion and the maximum inband FWM crosstalk. A comparison between the system spectral occupation and the signal-to-crosstalk ratio (SXR) versus the channel input power for the equal channel spacing and the proposed channel allocations validates the proposed solutions in the case of single-mode, nonzero dispersion-shifted, and dispersion-shifted fiber. For a 32-channel system, SXR improvements up to 4 dB without bandwidth expansion, and bandwidth savings up to 15 nm with a guaranteed minimum SXR of 25 dB, are obtained with respect to an equally spaced channel allocation.