Abstract
We study the structural and dynamical characteristics of the sodium atoms inside and outside the ``diffusion channels'' in glassy Na$_2$O-4SiO$_2$ (NS4) using classical molecular dynamics. We show that on average neither energetic arguments nor local environment considerations can explain the increased density of sodium atoms inside the subspace made of the channels. Nevertheless we show that at low temperature the mean square displacement of the sodium atoms inside this subspace is significantly larger than the one of the atoms outside the channels.