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A Gentle Introduction to Routing
October 8, 2008
Before we get into the details, a clarification. When you hear people refer to "non-routable addresses," they are talking about RFC 1918 IP addresses, i.e. private addresses. Despite the misleading label, they certainly are routable. You can and should have some 10.x.x.x networks for local access and management. They can even be co-mingled with your real routers. They are called �non-routable� because the Internet routers will drop them. You should drop these packets at your border, as was pointed out in this Border Security article last year. This is a point of confusion for a lot of people.
On to the topic at hand.
Routing, in essence, is the act of finding a path from one place to another on which a packet can travel. To find this path, we need algorithms. They will generally be distributed among many routers, allowing them to jointly share information. Routing is said to contain three elements:
Our installment on layers actually introduces a bit of routing by talking about the paths an IP packet takes through operating systems and routers. What may not have been clear, though, is how the routing table lookup step works.
We also need to understand some really basic problems with routing. Just like in Layer 2, routers need to be redundant. Redundancy always introduces the possibility of a loop, and every routing protocol has to deal with this. As we�ll see in future Networking 101 articles about specific protocols, this is pretty much a solved problem.