All devices on the World Wide Web are identified by a specific number called an IP address, for instance 220.127.116.11. When you have a site, the domain that you enter to open it is for your convenience, however the server where the website files are still has an IP. Since there're far more websites and devices than there are IPs, all of the shared website hosting servers have a number of websites under the same IP, while when you use a dedicated server you'll get a dedicated IP too. Even in the first case though, you're able to get a dedicated IP for your sites and host them on a shared server. One advantage would be that you will enjoy far better search engine positions since a dedicated IP generally means a faster loading website. Furthermore, you need such an IP when you want to obtain an SSL certificate for your website and secure the info that visitors submit on it.