[BaCkSeAtDrIVeR]

This router (and most routers provided by VSNL too) can be used as ordinary modems - you have to install and use PPPOE (ppp over ethernet) software - pppoe and pppoeconf on linux boxes, and rppoe on boxes running microsoft.
The configuration you are talking of - the router is running a DHCP server, and assigns a local IP address to your computer. The public IP address is assigned to the router. This is the right thing, if you are using microsoft OSes, because your computer is securer. But I use exclusively linux here, and I use the router as a modem and run a pppoe software, with the public IP address assigned to the computer, I have better control. One of the best things with this pppoe setup is that I can access the files on my computer like when I am on tour. I simply call up my home to switch the computer, and with some "little tricks", I can find up its IP address. Next, I login through SSH, and can read the file I want.
I have set this up very securely, that even if he gets the root password, a third person cannot login. I bet.