I always hear that static IP address works better on SEO. But I am not clear that why it works better. I think search engines always crawl your site by url not by ip address. If your url is stable, no matter what your ip is. Any one can bring me deep in this?
Don't worry about it, it is really a non-issue issue these days. Indeed, crawling, indexing and ranking is domain and URL-based, not IP-based. The vast majority of the Web is "dynamic" (i.e. on shared IPs); the number or possible dedicated, static IPs, although in the billions, is finite, and therefore the Web will continue to expand using dynamic IPs. What can matter and affect your SEO is the possible presence of dodgy, spammy or malware-laden Websites on the IP that you share. If there are a lot of such sites on your shared IP, Google may flag it as a "bad neighborhood" and all the websites on it may be affected. For that reason I recommend placing your site with a reputable hosting provider. An additional consideration is that all the sites on a shared IP share the server resources, and if server resources are limited, this can sometimes slow down your site's crawl rate and your page load time. There are some indications that page load time will eventually become a ranking factor, but we are not there yet. I hope this helps! By the way, you posted in the wrong forum, but I thought I'd reply anyway.
Thank you for the great reply! I unstand that crawl on dynamic IP will not be a problem. But may be affected by other sites that hosted on this server. I am just thinking to buy a dedicated IP for my website. Thank you again.