In a distributed wavelength-routed network, a lightpath request is blocked when its assigned wavelength is already occupied by another lightpath request. Conventional studies assume that a wavelength for the lightpath is selected randomly in the distributed lightpath setup method. However, this random selection method causes unnecessary blocks of lightpath requests, even when the arrival rate of requests is low. In this paper, we develop a novel method for assigning wavelengths, based on the first-fit algorithm. In our proposed method, the intermediate nodes forecast the wavelength that will be selected at the destination node, so that the subsequent lightpath requests avoid the forecasted wavelengths. The forecasted wavelength is thus kept available until the corresponding request reserves it, which prevents wavelength conflicts with other lightpath requests. Computer-simulated performance comparison showed that our method reduces the blocking probability by more than one order of magnitude compared to random selection.