All devices on the Internet are recognized by a special number called an IP address, for instance 220.127.116.11. When you have a website, the domain that you enter to be able to access it is for your convenience, however the server where your site files are still has an IP. Considering that there're many more websites and devices than there are IPs, all the shared website hosting servers have a number of sites under the very same IP, whereas when you use a dedicated server you'll get a dedicated IP too. Even in the first case though, you're able to obtain a dedicated IP for your sites and host them on a shared server. An advantage would be that you may get superior search engine rankings as a dedicated IP generally means a faster loading site. Furthermore, you need such an IP in case you want to get an SSL certificate for your site and protect the data that your visitors submit on it.