its with all GRPS service provider, not only AirTel / but BSNL GPRS and others too, the point came in to my attention when I read this thread.... *www.thinkdigit.com/forum/showthread.php?t=58035
I tested with Hutch / AirTel / BSNL !!!
it seems with GRPS they work like a big LAN or WAN !!! where when u use GRPS modem to connect, u actauuly get connected to the ISP internal network, where ISP shares one Getway (single IP address) among many GRPS user !!!!
So, possibilities are ur Internel IP (Check by command, ipconfig /all) and external IP, check from any web service, like *whatsmyip.org/ are different !!!!
Chances are that u would have an IP in rage of Second level IP or Private IP such as 10.xx.xxx.xxx with ur internel system IP, and a 3rd level IP like 220.225.82.33 (for an example only) with ur externel internet getway !!!