Though I am not an MTNL user, it happens to me on my Airtel connection too, when I set the modem in bridged mode and access 192.168.1.1 for my ZyXEL modem it somehow always connects to a Beetel 220BX modem, presumably because in bridged mode the modem just acts as router and routes data only. To avoid this I switched over the modem to a 10.x.x.x range and so far no more problems. Could yours be the same problem too?