Any line starting with 0.0.0.0 will automatically “fail” to find the page. For instance, 0.0.0.0 www.whatever.com will completely block that domain. It won’t block www.whatever.co.uk or whatever.com, so you’ll have to add one line for each top level domain. It’s great for blocking the worst ad networks (the ones that leave 6 clickjacks per page) 11

Pi-hole is worth looking into if you're into this