Once your network starts to get really big, or you start to consider 'the internet' as your network, you need tools which dynamically route your data. Sites are often connected to each other with multiple links, and more are popping up all the time.
The Internet has mostly standardized on OSPF and BGP4 (rfc1771). Linux supports both, by way of gated and zebra
While currently not within the scope of this document, we would like to point you to the definitive works:
Cisco Systems Designing large-scale IP Internetworks
Moy, John T. "OSPF. The anatomy of an Internet routing protocol" Addison Wesley. Reading, MA. 1998.
Halabi has also written a good guide to OSPF routing design, but this appears to have been dropped from the Cisco web site.
Halabi, Bassam "Internet routing architectures" Cisco Press (New Riders Publishing). Indianapolis, IN. 1997.
Using the Border Gateway Protocol for interdomain routing
Although the examples are Cisco-specific, they are remarkably similar to the configuration language in Zebra :-)