Link multiple networks together to form large networks

How to link multiple networks together to form extremely large networks? Although xbee supports 64K nodes in a network. What if I wish to create a larger size network?
Is it possible set up a link between coordinators of different networks?
I cant get started on my architecture design until I resolve this :frowning:

Maybe this is useful for you, http://www.digi.com/blog/networking/1000-radios-in-xbee-digi-k-node/