A unifying stress-strain model for physical networks (polymer melts) and for permanent networks (rubbers) is presented. It is based on three assumptions : (1) the uncrossability condition of real chains can be modeled by the tube concept; (2) the tube diameter is a function of the average stretch; and (3) the tube volume is invariant with respect to deformation. The model predicts that stress-strain behavior of dry and swollen rubber networks is completely determined by four material constants : the equilibrium modulus G(infinity) of the bulk network; the critical entanglement modulus G(e)* which is equivalent to the plateau modulus G(N) of the un-crosslinked parent melt; a finite extensibility parameter alpha; and a solvent-polymer interaction exponent beta. Predictions are compared with experimental data in elongation, and agreement is excellent. The Mooney-Rivlin constant C2 of unswollen networks with high crosslink densities is limited to roughly G(N)/2, and the origin of the C2 term is shown to be due to nonaffine deformation of the entanglement network. Nonaffine deformation of network strands is caused by an increasing lateral restriction due to neighboring chains, while upon swelling, the nonaffine reduction of the microscopic length scale leads to the vanishing C2 value of highly swollen rubbers.