So you are saying that as soon as modem is switched on, you are ready to surf, right?
Case I
When you open IE or whatever browser you use the very first time, do you get a "Connect ..." dialog box, which has your BSNL username, password and buttons - Connect, Cancel, Properties, Help?
In this case your modem is configured as a
Bridge Case II
No connection dialog as mentioned above. You are immediately connected to internet and your home page is displayed.
In this case your modem is configured in
PPPoE mode.
In Bridge mode, there is no need for port-forwarding as all ports are automatically forwarded. In PPPoE, you need to open the ports manually. Visit
PortForward.com - Free Help Setting up Your Router or Firewall for help on "How to".