All devices on the Web are recognized by a special number called an IP address, for example 188.8.131.52. When you have a site, the domain name that you enter in order to open it is to save you time, but the server where your site files are still has an IP. Because there are far more websites and devices than there are IPs, all shared web hosting servers have a number of websites under the same IP, whereas using a dedicated server you'll have a dedicated IP too. Even in the first case though, you will be able to acquire a dedicated IP for your sites and host them on a shared server. An advantage would be that you will get improved search engine positions as a dedicated IP usually means a quicker loading website. Also, you need such an IP in case you would like to acquire an SSL certificate for your website and secure the data that your visitors submit on it.