All devices on the Web are recognized by a unique number called an IP address, for example 220.127.116.11. When you have a website, the domain that you enter in order to load it is for your convenience, yet the server where the website files are still has an IP address. As there are many more sites and devices than there're IPs, all of the shared hosting servers have a number of websites under the same IP, while when you use a dedicated server you will have a dedicated IP too. Even in the first case though, you will be able to obtain a dedicated IP for your websites and host them on a shared server. An advantage would be that you will enjoy far better search engine positions because a dedicated IP generally means a quicker loading site. In addition, you need such an IP in case you intend to acquire an SSL certificate for your site and shield the info that your visitors submit on it.