Six oxide and oxynitride glasses were synthesized in the Y-Mg-Si-Al-O-N, Nd-Mg-Si-Al-O-N and La-Mg-Si-Al-O-N systems. As already known, nitrogen introduction increases the T(g), packing factor and mechanical properties of the glasses. Cationic substitution also has an influence on the glasses' behavior, particularly in terms of sensitivity to indentation load/size effect (ISE). The structure of the yttrium-containing glasses was investigated by mean of (27)Al and (29)Si MAS-NMR. Al is found to occur for 2/3 as a network former and for 1/3 as a modifier. The oxide glass mainly contains Q(2) and Q(3) silicate units and SiO(3)N and SiO(2)N(2) units are created when nitrogen is introduced into the glass network. The average number of rigid bonds per network former < n > was calculated from the glasses' composition. A discrepancy between < n > and the Raman spectra of the glasses suggests that parts of the magnesium behaves as a former in the network. < n > seems to be a key parameter governing hardness and sensitivity to ISE and can be linked to normal/abnormal behavior of glasses regarding indentation.