Extensive molecular dynamics simulations have been performed on sodium disilicate glass Na2Si2O5, using realistic empirical interaction potentials. The structure of the simulated glass agrees reasonably with experimental diffraction data. Analysis of this structure is performed by a number of different methods and the results provide support for the modified random network model. Quantitative values of the sodium diffusion coefficient over the temperature range 1000-2000 K yield a diffusion activation energy which is close to the experimental value.