BungeeCord

BungeeCord is a special API for CraftBukkit that allows you to link multiple servers together with ease. Most high-occupancy servers use this. It is one of the most convenient ways to have multiple servers and use all of them with one ip address. It was created by md_5 and is on SpigotMc.org. It's very simple to link servers together with this API. All you will need is Notepad++. If you are not running it from a local computer, normally the host you are using will set it up for you.