Minecraft's main man -- Notch -- has tweeted that server interlinking via portals will come to Minecraft at some point in the future. Essentially, this would make bouncing back and forth through separate worlds feasible in multiplayer sessions, paving the way for a near-limitless mining landscape. The possibilities are nothing short of mind-boggling.
Yes, you will eventually be able to link servers with portals[...]
Continue reading: Server Linking Via Portals Coming To Minecraft...
