The form factor of a labeled chain randomly cross-linked into a network in a tubelike approach is presented. Within a harmonic approximation for the constraining potential, the action of cross-links and entanglements is modeled by an effective tube, which is deformation dependent. The only parameters of the model are the reduced microscopic deformation, following a power law in the strain, and the diameter of the tube; which is inversely proportional to the strength of the constraints. First experimental results on polybutadiene networks with varying cross-link density are discussed. The conclusions drawn confirm a less than affine deformation at the molecular level while segmental fluctuations are found to be unperturbed despite the network topology and of the same size as obtained from theology.