Hello Friends, I read a article that we do not registered domains in same IP, but i want to know that why this is not good for websites?
Just to be clear, a domain registration is not associated with your IP, the IP is specific to your hosting. There are numnerous sites on the same IP anyway if you have shared hosting, so this isn't a problem, the only time it can be is if you have multiple sites on the same IP all linking to each other, as the links COULD be devalued, so you would therefore not get the benefit of the links. Although I find it a bit concerning that you have SEO links in your sig and are asking a simple question like this!
I think you're confusing domain registration and web hosting. You can host many sites on 1 IP without affecting your SEO, the only time it may be an issue is if google has blacklisted any of the sites on that IP block.