Know Your LAN (Part Two)

Local Area Network, or LAN for short. The whole Internet is based on the concept of LANs, literally.

Part One - What is a Switch

Part Two - How and what does the Switch do? (You are here)

Part Three - Advanced explanation of the Switch from a Network Engineering viewpoint. (Link to be added)

Although not common, there are those who believe you must have a Router in order to have a LAN. That is incorrect.

A LAN by definition, is a connection between two or more devices. All you need is a Switch and two PCs and you have a LAN. The only issue is, that LAN of yours has no access to the Internet. With that out of the way, lets start this section.

Some of you are wondering "Why in the world can't a Switch go to the Internet?". And that's a very good and valid question!

PCs have a name (IP Address) and social security number (MAC Address), right? To access the internet, you need to use your name. The thing is, your Switch does not understand what a name (IP Address) is. The Switch works solely with your social security number.

Put it this way: Imagine you're going out and meeting a person for the first time. Then suddenly you say this:

"Hello, I am 220AB244920AA"

Makes no sense right? However, the Switch uses only your social security number. It is unable to use names as identifiers.

Now you're probably thinking "Why can't it?!". Another good question! The reason this time however, is because it's simply not smart enough, with exceptions.

There are in general two types of Switches: Ones that do not understand IP Addresses and ones that do. Since this is not the advanced section, I will not cover the latter, only the former.

So, the answer to "What does the Switch do?" is very simple. It connects two or more PCs together but, with no access to the Internet (until a router is added to the LAN).

The only remaining questions is "How does it connect PCs?".

Switches have tiny agents that gather information about each device in the network. Those agents are called ARP (Address Resolution Protocol) agents. Their job is to gather information about each device in the network and give that information to the Switch in order for it to create a MAC Address Table. ARP agents basically come knocking on every PC's door asking for their information.

What's interesting, is that it's almost like real life. What happens when you go live at another house? Your home address changes, and your friends and family end up asking for your new address, right? ARP agents come and go periodically exactly for that reason!

After a certain amount of time, the ARP agents go out asking for your information again. Checking, to see if you still live at the same address (one of the switch's ports) or someone else has taken your place!

The Switch memorizes which MAC Address lives off of which port. Then based on that table, makes the decision of where to send the data.

At the end of this post, I would like to give a small introduction of some basic things in networking.

When your PC sends data, something called a "Frame" is created. That frame holds the Source MAC Address (You) and Destination MAC Address (your friend), among other things. The Switch makes forwarding decisions based on that frame's Source and Destination MAC Addresses.

I hope you enjoyed reading this, and see you next week with the advanced stuff. Next week's post will be a bit longer since it covers more stuff.