Is it possible to block all the IP addresses from an Internet Service Provider?

There are billions, billions of ip’s. there is one for each cell phone, computer and ipod. You may as well just have the site secret so that only you can get to it and some sort of login. If you want to block a certain area there is ip blocking like that. I’m not sure why you want to block all ip’s though.

This can be done if you can determine the ISP’s subnet. This can be determined by doing a WHOIS query on the ISP’s domain name and then querying the ISP’s nameserver for the IP address of some domains known to belong to the ISP.
For example, I just did a “whois comcast.net” and got a reply containing the names of several nameservers. I picked one and did “dig @dns105.comcast.net” which gave me a bunch of A records all in the 68.87.0.0 block. It looks like the block is probably 68.87.0.0/16 but it’s hard to tell the size.
ISP’s are all registered with their regional authority. If you need to find the netblock of an American ISP contact ICANN. They may or may not help you, I haven’t tried.
Once you have the block of IP addresses, it’s simple to craft a firewall rule to drop packets coming from or going to there, assuming you have a real firewall.
It’s possible that the ISP you are dealing with may not be a “real” ISP but a company leasing address space from a larger pool, in which case their addresses may not all be from the same block.

Blocking a huge list of IP addresses would be very inefficient, but you can resolve the hostname from the IP address using a scripting language such as PHP.

$block = ‘embarqhsd.net’;
$host = gethostbyaddr($_SERVER[‘REMOTE_ADDR’]);
if(strpos($host, $block) === true) exit();

ummm…what and why are you doing this?
What is the issue or reason?