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The problem of determining the number of spatial degrees of freedom (d.o.f.) of the signals carrying information in a wireless network is reduced to the computation of the geometric variation of the environment with respect to the cut through which the information must flow. Physically, this has an appealing interpretation in terms of the diversity induced on the cut by the possible richness of the scattering environment. Mathematically, this variation is expressed as an integral along the cut, which we call cut-set integral, and whose scaling order is evaluated exactly in the case of planar networks embedded in arbitrary three-dimensional (3-D) environments. Presented results shed some new light on the problem of computing the capacity of wireless networks, showing a fundamental limitation imposed by the size of the cut through which the information must flow. In an attempt to remove what may appear as apparent inconsistencies with previous literature, we also discuss how our upper bounds relate to corresponding lower bounds obtained using the techniques of multihop, hierarchical cooperation, and interference alignment.