A model for ionic transport in glasses is proposed assuming that, like for ionic crystals, the displacement of cations would result from the migration of interstitial pairs. The formation of these pairs is supposed to be an activated function of the temperature and their migration to be dominated by an activated jump mechanism at low temperature and by a free volume co-operative mechanism above the vitreous temperature. Using numerical fits for conductivity data in the whole temperature range, that change of mechanism enables the determination of the defect formation enthalpy and its migration enthalpy in the low temperature domain.