Dedicated Servers come with one IP address. It is possible to order additional IP addresses through the 1&1 Control Panel. When ordering additional IP addresses, 1&1 ensures that the IP is set to route correctly to your individual server however it is still necessary to add this IP address to the operating system to ensure the server knows which additional IPs are available to use.
For help on how to order additional IP addresses from the 1&1 Control Panel, please reference How do I order additional IP addresses for a dedicated server?To add additional IPs that you've ordered to your server, you will first need to log into the server via Remote Desktop. Even if you have Plesk, you will still need to add the IPs via Remote Desktop.For help on how to connect to your server via Remote Desktop, see How do I log in to my Windows Server via Remote Desktop?