I think your 'parent' ISP is one of those braindead ISPs, who configure their DNS servers so that when they get a query for a non-existing domain name, they send you to some ad site (instead of replying with a "no such domain" message, as they should). Just surf to that IP address and it should be obvious. Sadly, this is becoming more and more standard practice...
So basically, there's nothing wrong with your set-up.
Best thing to do is complain to your ISP, tell them they violate RFC standards and engage in the general detriment of the world wide web; or even better switch to another ISP and tell your current one why you took your business elsewhere
Best regards,
nukkel