National IP/MPLS network have being designed on top of fixed-grid DWDM optical networks so to spread out covering large areas. Besides, multilayer IP/MPLS-over-DWDM networks take advantage from grooming to achieve high spectrum efficiency, filling the gap between users' flows and wavelength channels' capacity. The advent of the flexgrid technology providing a finer granularity makes possible to perform grooming also at the optical layer. We propose to flatten previous multilayer approaches and advance towards single layer networks consisting in a number of IP/MPLS areas connected through a flexgrid-based core network. A two-step procedure to design such flexgrid-based IP/MPLS national networks is proposed. In this paper, a future large flexgrid core network inter-connecting small areas is shown.