You can get most answers on your questions with a simple Google or forum search. An IP-adress is built like this AAA.BBB.CCC.DDD. Since many spammers had several sites linked together on same host for higher link juice, many search engines decided to ban linked sites coming from the same C-ip. So if you have 12 sites with the same C-ip and linked them together, it wouldn't benefit you as much as with 12 unique.