Routing and bandwidth assignment of new paths of different bandwidths, occupying different numbers of adjacent frequency slots in a wavelength switched optical network, involves selecting a route, and assigning a set of adjacent frequency slots. The assignment can place wider bandwidth ones of the new paths at an opposite end of a spectrum of the available frequency slots, to an end where narrower bandwidth ones are placed. A size of sets of available adjacent slots remaining after the assignment is likely to be increased, compared to a conventional first fit assignment. A wider subsequent new path can sometimes be accommodated along all or some of the route and thus the blocking probability can be lowered. The selecting of which of the possible routes to use can be made dependent on which has more sets of available adjacent frequency slots, or which has a wider gap between occupied slots.