1. A method comprising the steps of: implementing by a computer an embedding of cloud demands over a software defined flexible grid optical transport network, the implementing comprising: determining required resources for each node and arranging virtual nodes in the network in a descending order of a total number of required resources; mapping the virtual nodes over physical nodes while balancing a load for different types of resources; estimating spectral resources for each link and arranging virtual links in a descending order of estimated spectral resources; mapping virtual links over physical routes and selecting a modulation format for the virtual links while balancing the load over physical links; assigning wavelength and spectrum to the virtual links; and selecting a virtual node VN from the top of a list, determining a set of feasible modulation formats for each of a k-shortest routes connecting physical nodes PNs on which virtual nodes VNs are mapped based on their reachability requirements and the requested line rate; finding a bit-map of each of the k-shortest routes connecting PNs on which VNs are mapped, a bit-map of a route being determined by performing bit-wise logical end operations on the bit-maps of all physical links along the route, for each modulation format; determining the total required spectrum, determining a probability of mapping the VL on a route using a modulation format based on a ratio of the number of wavelength slots starting from which consecutive wavelength slots are available for a modulation format on the bit-map of a route k to the total number possible wavelength slots starting from which consecutive wavelength slots can be mapped; and selecting a route and modulation format that has a maximum probability for the virtual link VL, and checking whether the maximum probability is 0 or not.