The Border Gateway Protocol (BGP) currently in its version 4 makes the Internet redundant – this I have always found fascinating! It dynamically routes traffic between Autonomous Systems on the Internet, and supports Classless Inter-Domain Routing (CIDR).
In 2000 I was offered to buy bgp4.com from O3 Networks in Amsterdam, and jumped on it, even though I had to outbid Lucent – and I never regretted it (quite on the contrary). While very few understand the concept (it is a geek thing, admittedly), those who do, “get it” … :)
The 2006 RFC 4271 describes the basic standards protocol.
There are many ways to deploy BGP4 in public and private networks – which makes it so interesting; the main purpose being optimal and fully dynamic routing for best connectivty with the lowest latency, and the ability to work on latencies by adding private peering agreements, when the public ones are insufficient.
A side benefit is the ability to use intelligent BGP4 routing to fight large-scale distristibuted Denial-of-Service (dDoS) attacks; there are many forms for other, more sophisticated dDoS attack vectors, but BGP4 can greatly help with the multi-Gigabit attacks occuring all the time.
If you like to explore this more, there are excellent additional ressources, andvideo tutorials out there, such as here and here (Router Gods).