Chapter 2: Internetworking Basics

Note: I have discontinued this blog. Instead I have started a FREE video series on CCNA in YouTube. You can find it here. 

Alright, yesterday we saw a bit about networks and IP Addresses. Today we will see in detail how a network device communicates with each other.

Before we start, let us define COMMUNICATION.

What is communication?

Good Question.... well...

DEFINITION TIME: Communication is the process where two or more devices pass information between themselves and is accepted.

Communication can be divided on the basis of destination.

1. Unicast ---> One to One
2. Multicast ---> One to Many
3. Broadcast ---> One to ALL

Based on characteristics of communication, it can be divided as:

1. Simplex ---> Communication possible only in one direction
2. Half Duplex ---> Communication possible in both direction, but only one device at a time
3. Full Duplex ---> Communication possible in both direction simultaneously

Let us start by taking a very basic network with a hub and two PCs as given in the diagram below:

DEFINITION TIME: A HUB is a multi-port repeater, sending out the same signal onto all the other ports.

Lets assume Jack wants to say a HELLO (or whatever packet of information) to Jill. What do you think Jack will do? Any guesses? Did you say it will send HELLO to the Hub addressed to Jill? NO. Then? Do you think it will send HELLO to Jill's IP Address? Sorry Again! Actually the answer: To its MAC address! A hub works on the physical layer (Layer??? Dont worry, we will talk a LOT about layers in tomorrow's chapter!)

MAC Address, what is that?

DEFINITION TIME: A MAC (Media Access Control) Address is the hardware address. It is also called Burn-In-Address. This address cannot be changed and is unique to the NIC. It is a 48bit HexaDecimal number

Right... so how does Jack communicate with Jill? If Jack know's Jill's MAC Address, it just sends the message to Jill's MAC address. If it knows only Jill's IP Address or hostname, it can send a broadcast, asking Jill for her MAC address! Jill receives the broadcast and sends a reply back to Jack with her MAC address. Jack then uses that information and sends the message to Jill.

With this given configuration (connecting through a hub), it is a pointless exercise, because even after addressing the message to Jill's MAC address everybody connected to the hub will receive the message! 

This is because a hub has only one Broadcast Domain and one Collision Domain.

What is a Broadcast Domain and a Collision Domain?

DEFINITION TIME:  

Broadcast Domain is the space on the network where a broadcast reaches

Collision Domain is the space where only one device can communicate at a time

So if two or more devices connected to a hub, communicates at the same time then a collision occurs. So as more and more devices get connected to the HUB, the network will start crawling.

A better network topology would be to use a SWITCH instead of a HUB as shown in the diagram below:

Really? A Switch?

DEFINITION TIME: A SWITCH is a multi-port device, which breaks the collision domain into the number of ports it has. It still has only 1 broadcast domain.

What the above definition means is that if the switch has 4 ports, it has 4 collision domains. That means 4 devices can communicate at the same time without collision. If any device sends a broadcast signal, every device connected to the switch WILL get that since it has only 1 broadcast domain!

But how does this configuration change anything from the previous configuration? Well for one, after Jack gets Jill's MAC address, Jack can start communicating exclusively with Jill, instead of his message being sent to all the devices connected to the switch! Like mentioned above, as the network grows with more devices, there will be so many collisions happening that the network will be literally JAMMED! A switch improves performance and also the bandwidth usage.

In addition to the differences mentioned above, one of the main differences between a hub and a switch is that a switch is an intelligent device. That means, as communication takes place, a switch will start learning MAC addresses of the devices connected to it and start storing it in its MAC ADDRESS TABLE. It also notes into which port a particular MAC address is connected. So the next time if any device wants to send a message to a particular MAC, the message will be automatically sent to that particular device!

Now let us assume that you have devices from different departments connected to this switch. So when any device sends a BROADCAST, all the devices receive these messages. Even if they know it is not meant for them, they will have to process a broadcast message! This is waste of precious device time and bandwidth. This problem will start magnifying as your network grows! So to break a large broadcast domains into smaller domains, we use a ROUTER. Lets have a look at the following diagram:

Please define a router.

DEFINITION TIME: A ROUTER is a device used to communicate to different networks. A router breaks the broadcast domain and collision domain, equal to the number of ports on it.

Suppose Jack and Jill are from HR and Tom and Sam are from Accounts, their devices need not send broadcast messages to each other. A router is a device used to communicate between networks/departments. They are also used to connect to the INTERNET.

The internet is made up of a lot of ROUTERs. These routers are so intelligent that they 'learn' about the routes to any network in the world. They use protocols to do this as you will learn in later chapters.

Alright, that will be all for the day!

Summary

We have learnt internetwork basics today. Let us summarize:

1. Definition and Types of Communication
2. Different network devices like HUB, SWITCH, ROUTER
3. Differences between these devices
4. Definition of Broadcast Domain and Collision Domain

Fastest Finger First

A) A HUB has ..........

1. Unlimited Broadcast and Collision Domain
2. 1 Broadcast and 1 Collision Domain
3. Collision Domain Equal to the number of ports and 1 Broadcast Domain
4. Broadcast Domain Equal to the number of ports and 1 Collision Domain

B) A SWITCH has ...........

1. Unlimited Broadcast and Collision Domain
2. 1 Broadcast and 1 Collision Domain
3. Collision Domain Equal to the number of ports and 1 Broadcast Domain
4. Broadcast Domain Equal to the number of ports and 1 Collision Domain

C) A Multicast means.........

1. One to many
2. One to All
3. One to One
4. Many to One

D) A Router is used for........

1. Connecting devices from the same network
2. Connecting devices from different network
3. Broadcast to all the port
4. All the above

Thank You!

If you have any queries, post it as a comment below. I will answer all your queries. Also as soon as you complete today's class, give the correct answers for FASTEST FINGER FIRST in the comment section below.