I'm behind a router and I want to host, how do I do it?

You'll have to forward your UDP ports range from 49000 to 65000, if you have no idea how to do it then google for "[your router's name] port forward" the name of your router should be written on the router itself

You must be sure that you configure your port forwarding on your router to your local IP address:

192.168.1.2 = Your PC IP Address

Forward UDP - 49000 > 65000 to 192.168.1.2

Just curious - why such a large range?

I don't have any control of it, flash player automatically chooses a random port =/