The DNS for either ISP may differ, the ISP could block an offensive or competitors web site.
Your skill at the use of NSLOOKUP or TRACEROUTER is required. I only note the names of the tools which you teach yourself about with google.com.
Bob
I have two different IPS's. My browser(IE6) will connect to one IPS and not connect to the other one. I can sign in and connect to both of the providers. What would keep the browser from locating the entered address in one and not the other?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic