bascule has the right idea. The hardest part about changing or upgrading the internet is dealing with the so-called "default free" zone of the Internet. This part of the Internet, which includes most of the backbone routers and NAPs where different providers exchange traffic, contains routers that do not have default routes. In other words, they have a table of all of the CIDR address blocks for all of the networks on the entire Internet. This routing information is maintained by the BGP protocol.
