Now, find your public IP, and voila!

Similarly if I walk into a coffee shop and use the wireless LAN, I have the same problem - my IP address isn't globally accessible. Why do we care?

At present most campus nets are joined with the national education network, which can't satisfy the user's need be caus the speed to access the public is slower.

Each of those kinds of machines may have multiple IP addresses, some public and some just for use on the local network.

Video chat devices on the LAN with good results, but in the public network is not very good!