BungeeCord
BungeeCord, the 6th in a generation of server portal suites
...The project is maintained by SpigotMC and has long been used by server owners who operate lobby, minigame, survival, creative, and specialized game-mode networks. BungeeCord works as a layer between players and backend servers, forwarding connections and managing server-to-server transitions. It also provides an API that developers can use to create plugins and extend proxy behavior. Its main value is helping Minecraft communities scale beyond a single server while keeping navigation smoother for players.