The Layers of the Internet - Part 1

The Internet is a complex place. There are many articles about the history of the internet, and it's a little beyond the scope of what this blog is about, so I won't go into it too much.
Suffice to say, the only way the internet works is through rigorous adherence to global standards. These standards were developed over the 15-year genesis of the Internet from research labs to commercial use.

One of the best ways to describe how these standards, and indeed the entire internet, works is known as the "layer" method.

Layers are a great way to explain many things, and the layers that we will be looking at today not only apply to networking, but to almost every form of computing or digital signal processing.

There are four layers in the "standard" Internet topology:

• Layer 1: Link Layer. This is the physical link between devices
• Layer 2: Internet Layer. This is the "virtual" layer where the data moves around networks
• Layer 3: Transport Layer. This layer defines how data moves around devices
• Layer 4: Application Layer. This is the layer that shows how data is shared between programs.

Those short little descriptions probably mean very little to most of you, so let me break it down a little bit more.

The Link Layer
When I think of the best way to describe the Link Layer, this image comes to mind:

What you're looking at there is a fairly typical "Distribution" switch, and a lot of optical fibre.
The Link Layer is the only physical connection layer in the Internet Protocol. It defines all the different ways that you can connect devices together if you want them to be on the internet.
If you put your mind to it, you could easily rattle off a lot of the different standards that are in the Link Layer, for example:

• Wireless Networking 
• Ethernet networking (a.k.a. "Blue String" - those blue cables that we are all familiar with)
• Fibre Networking 
• ADSL (Asymmetric Digital Subscriber Line - The way most of us get our Home internet)
• 3G/HSDPA - The wireless Broadband that most of us use on our phones.
• DOCSIS, a.k.a "Cable Internet" - networking over Coaxial cable, similar to Cable TV (Thanks djzort)

The list goes on. The greatest thing about the "Layer" system of the Internet is that it doesn't matter how you connect devices together at the Link Layer, so long as they follow the Link Layer Standards. As soon as you have that "blinking light" that shows you are connected then you can start passing information around the Internet Layer.

The Link Layer extends across the entire Internet. Just ponder on that for a second; every device that is connected to the Internet is in some way, shape or form, connected. The Link Layer is the only layer at which every device is connected; once you start moving into the "Virtual" layers (layers 2-4) you start segregating devices into separate virtual networks (or "subnets"). But for now, let's just muse on the topic of every device acting together in synchronism.

The last thing that I will mention about the Link Layer is the address that applies to it. Obviously there is no point in connecting every device on the planet unless you knew which one you wanted to talk to. Therefore every device that connects to the internet has a unique address. This address, known as the Media Access Control (MAC) address, is a unique number assigned by the manufacturer.

The MAC Address is a 48-bit number (that is, 48 "1's" or "0's"), which makes for about 300 Trillion different addresses. Every single device that is capable of connecting to a network has a MAC address. This laptop, for example, has two addresses; one for the Wireless connection and one for the hard-wired connection. Even so, the IEEE doesn't expect that we'll run out of MAC addresses this century.


We'll look at the Internet Layer next time (probably in a couple of days). I thought it best to break things up for now.