All devices on the Web are identified by a special number called an IP address, for instance 220.127.116.11. In case you have a site, the domain name that you type to open it is for your convenience, but the server where the site files are still has an IP address. Since there're a lot more sites and devices than there're IPs, all shared web hosting servers have a number of sites under the same IP, whereas with a dedicated server you'll have a dedicated IP as well. Even in the first case though, you're able to obtain a dedicated IP for your websites and host them on a shared server. An advantage would be that you'll enjoy far better search engine rankings as a dedicated IP usually means a faster loading site. In addition, you need such an IP in case you plan to buy an SSL certificate for your website and secure the info that visitors submit on it.