All devices on the World Wide Web are recognized by a unique number called an IP address, for instance 126.96.36.199. When you have a site, the domain that you enter to access it is for your convenience, yet the server where your site files are still has an IP address. As there're far more sites and devices than there're IPs, all shared web hosting servers have a number of websites under the same IP, whereas with a dedicated server you'll be given a dedicated IP as well. Even in the first case though, you're able to acquire a dedicated IP for your sites and host them on a shared server. One advantage would be that you'll get far better search engine rankings because a dedicated IP generally means a faster loading website. Also, you need such an IP if you want to get an SSL certificate for your site and shield the info that visitors submit on it.