Additional IP addresses can be ordered once your server subscription is setup. This is done through your Billing and Support panel at https://secure.servadmin.com/cp/. Once the additional IP addresses are allocated to your server you will need to set them up as Ethernet aliases.
Adding IP addresses in Plesk
You will have to login to the Plesk control panel as the admin user. After logging in click on the "Servers" link on the left menu. Then under the "System" category on the main screen click on the "IP Addresses" icon. Under tools there will be an icon to add a new IP address.
Adding IP addresses in DirectAdmin
You will have to login as the server administrator. From the main Administrator menu, click the "IP Manager" link. Then you will be able to add the IP address using the form at the top of the page.
Adding IP addresses in Webmin
After logging into the control panel you want to click on "Networking" section. Then click on the "Network Configuration" link, and then click on "Network Interfaces". IP addresses can be added by clicking on the "Add a new interface" link.
The netmask for all IP addresses should be 255.255.255.0
IP addresses can be added through the command line, but it differs slightly for each operating system.