Hi Arihant,
ISP stands for Internet Service Provider. That is who you pay to have internet service in your home or business.
I think what you are asking about is an IP address. IP stands for Internet Protocol, and is a number that is assigned to your computer for your connection to the internet. Some folks have a static IP, and others are dynamically assigned by their ISP.
The problem with banning people by IP is that so many of us don't use a static IP, but instead have a different one each time we log on to the internet. Most of us are unaware of our IP address. These numbers simply help identify a machine on a network.
So, now this forum allows us to ban folks in several ways, one of which is IP, but also by user name, by host name, and by email address.
Here's more info on IP addresses:
http://en.wikipedia.org/wiki/IP_address