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Video - Network Layer (Layer 3) of OSI Networking Model

Transcript + My Comments

This video by Kevin Wallace you learn about the Network Layer of the OSI networking model. Keven's videos teach networking in a light conversational way, with emphasis on retaining the information for the purpose of passing the CompTIA Network+ certification exam.

At the Network Layer (Layer 3) of OSI Networking Model we're concerned with:

• Logical addressing
• Switching
• Route discovery and selection
• Connection services

Logical addressing; It's here where we logically assign a network address. We have different routable protocols like IP (Internet Protocol), Novel's IPX, and Appletalk. But today Novel and Apple are using IP primarily to route their traffic, so IP has become by far the predominant layer 3 protocol.

You might be surprised to see the term "switching" when we talk about layer 3 because we think of switching typically as a layer 2 concept. But switching can mean "packet switching", that's something that does happen at layer 3.

With packet switching, our data is divided into packets, and these packets have a header saying where the packet came from, and where the packet is destined. And a device called a "router" is going to intelligently switch - it's going to "packet switch" that packet towards its destination.

At layer three there's also the concept of "circuit switching", that's where we can dynamically bring up a circuit on demand and then tear it down when we're done. Think of that as being somewhat analogous to placing a phone call.

At layer three there's also "message switching". Here our data stream is divided into different messages, and these messages, much like a packet, have a destination address. But they're not necessarily immediately sent to the destination address.

It's almost like we're sending email from one email server to the next, and the next, in other words the next hop device on the way to a destination, a message might be stored. And on the next device it might be stored for a while before it's forwarded. This store-and-forward approach to forwarding messages through a network is not great for real-time traffic obviously.

Primarily though, in the networking world, we're concerned with packet switching. We have source and destination addresses on our packets and we packet switch those packets. In other words we route those packets towards their destination.

Something else going on at level 3 is route discovery and selection. We said that a device called a router resided at layer 3 and that router is going to make intelligent forwarding decisions.

Remember how a switch works down at layer 2? At layer 2 a switch made forwarding decisions based on physical MAC addresses. However, with routing at layer 3, we're making forwarding decisions based on destination IP addresses typically.

How does a router learn how to get to specific destination IP addresses living in specific destination IP networks? Well there are a few ways.

We might have a network that is directly attached to a router interface. By definition a router knows how to get to this network - the router is part of the network. We could alternately go in and statically configure a route.

We could say to get to this network, you need to go to this interface, or you need to go to this next hop IP address, or we could run a routing protocol, something like OSPF, RIP, or EIGRP. A dynamic routing protocol can educate the router as to how to reach a destination network.

• OSPF (Open Shortest Path First)
• RIP (Routing Information Protocol)
• EIGRP (Enhanced Interior Gateway Routing Protocol)

So those are the three primary ways of learning routes and selecting routes inside a router.

• directly attached networks
• statically configured networks
• dynamically learned network

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