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SDN - Overview
SDN stands for 'Software Defined Network'. When we say 'Software Define Something', it usually mean a system or a component implemented in software and in conventional way, that system/component used to be implemented in Hardware or configured by human intervention (manually). Applying this to SDN (Software Defined Network), the term SDN implies that we want to implement a Network (or Network Components) by software and those component used to be implemented hardware or configured by human intervention.
Then you may ask 'which part of Network will be replaced by Software in SDN ?' How ? and Why ?
Just keep these questions in mind and follow along following sections and the answer gradually emerge.
What is the typical structure of current network ?
Before we talk of the new system (SDN), let's briefly think of how we implement and configure the network in current (conventional) way.
In a very simple representation, a network can be represented as a lot of switches (routers) connected together to form a fishing net or spider web like architecture.
Each componet (Switch in this case) is made up of roughly two parts. One part is for forwarding a packets from a port to another port and the other part is for setting up various information/configuration that directs how those packets should be forwarded.
How do we configure (change configure) in conventional (non-SDN) network ?
Now let's suppose that we need to change the configuration of a network to optimize it for a specific purpose or specific use model.
How do we configure (change configure) in SDN network ?
 Stanford Seminar - Software-Defined Networking at the Crossroads (YouTube)