I want to create an Arduino with an ethernet shield that will render a router unable to work. Kind of like a DOS attack but inside the network. I have no idea how to communicate with the router and send information to confuse it and stop its correct functionality.
We don't generally help out with this kind of stuff. You are either a legitimate pen tester, in which case you certainly wouldn't be asking like that, or you are some kid trying to do nefarious things which isn't what we are into.
I suggest you do some studying on how routers work.
xtfishburnx: I have no idea how to communicate with the router and send information to confuse it and stop its correct functionality.
So you have no idea of what you are doing and consequently no idea of how much damage you will cause or how much it will cost others to put right.
I hope no-one gives you even a clue as to how to go about this.
You can do this to a hub very easily but a router will route around your interference with little to no impact on anyone else on the same router.
For a hub, just plug one port into the other.* No Arduino required.
*The more experienced types here will notice that this isn't a complete instruction although it does describe what you do.