The structures of a wide range of fast ion conducting glasses have been modelled using the reverse Monte Carlo (RMC) method, based on neutron diffraction, X-ray diffraction and EXAFS data. The glass structures typically consist of a 'network' component, e.g., phosphate chains, and a 'salt' component. These two components are strongly interlinked. There is no evidence that the salt component forms, to any significant extent; nanocrystalline clusters similar to the equivalent fast ion conducting crystals. The main effect of salt doping is to expand the network, leading to an increase in the accessible free volume and hence the number of conduction pathways. There is a strong empirical correlation between network expansion and increase in conductivity. However salt doping also increases the number of charge carriers and changes the average cation environment, thus decreasing the activation energy and increasing the cation mobility, and these effects cannot be separated from the network expansion. Increased salt doping reduces the number and strength of network cross links and weakens the glass structure, eventually leading to crystallisation.