# Internetworking

Welcome to Introduction to Internetworking
In this introductory section of the networking tutorials, you will learn and be able to :
* describe the purpose and functions of various network devices
* Select the components required to meet a network specification
* use the OSI TCP/IP Models and their associated protocols
* describe operations of OSI and TCP
* differentiate between LAN and WAN
Let's get started!

## Internetworking

The word, Internetworking, definitely brings to your mind much information. I am sure the first to come up will be the Internet. Yes! The Internet is also possible via Internetworking.
In our daily lives, internetworking has become part of us. You took a picture on your way home from school and try to share it with your friend. To do that, you will need to connect to your friends device, be it phone, computer or camera. Before the transfer of the picture actually happens, there should exist an agreement between your device and your friend's device. The devices must be able to talk to each other, so to speak. By so doing, your devices are able to share information, in this case, you are able to send to your friend that picture.
Internetworking is a combination of the words inter (which also means "between") and networking is the practice of connecting a computer network with other networks through the use of gateways that provide a common method of routing information packets between the networks. The resulting system of interconnected networks is called an internetwork, or simply an internet.
Please don't freak. The terms used in the above definition will be taught and explained in details throughout this course.
Internetworking became very important and have grown exponentially over the past 15 years. Internetworking had to evolve so as to be able to keep up with the huge increase in basic user needs such as sharing data and printers as well as more advanced demands such as video conferencing.
Now, lets consider how computers talk to each other. Its a very simple process and very fascinating.
Now, please refer to figure 1.1. How will computer name Rex communicate with Edna? Well, the two computers are all on the same Local Area Network. However, Rexford, cannot just send something out to Edna just like that. The first thing Rexford will have to do is to grab the Media Access Control address of Edna. What for? Simply put, without that, Rexford cannot send anything to Edna. It is the Media Access Control that ensures conversations or data transfer occur between intended parties only. And, fortunately, no networking device on earth today exists that hasn't got a MAC Address. In fact, MAC addresses are like names in the networking world. Have you ever wondered how you will get post mails if you had no name and your name was nothing? In like manner, networking devices use the MAC to know exactly where a particular information is coming from and where it is going.
The good thing of it all is that, there is the Domain Name Service (DNS) to help resolve hostnames into IP address. However, in the scenario shown above, since Rexford and Edna are on the same LAN, Rexford will just broadcast to Edna asking her for the information in which case DNS isn't needed.

## Network Segmentation

No matter how big a network might be or seem like, it is segmented. There are levels in networks. Every level of the network performs certain functions that are dedicated to it. Even if other devices in different levels can do similar jobs, such tasks are best left for the dedicated devices. In connecting computers together or internetworking in the olden days, it used to happen via a dump networking device called the Hub. This tutorial would not actually delve into Hubs since they are basically not used in production environments are very dump when compared to today's networking needs such as speed and security.
With the segmentation, it happens via the use of devices like routers, switches, and multilayer switches
Before we get into the various works and functions of these segmenting devices, I will like to bring to board basic explanations on these: Broadcast Domain and Collision Domain

#### Broadcast Domain

A broadcast domain is a logical division of a computer network, in which all nodes or devices or computers can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments.
I am sure you are familiar with the word, to broadcast. Well, it does not hurt if you find out in a dictionary like en.wiktionary.org. However, the basic idea behind broadcasting is sending something out without restrictions. Thus, in networking, we consider a logical (not literal) division of a network whereby it is possible for devices to send information to every other device in the network to be having a Broadcast Domain.
Don't force yourself too much to understand. During the span of this tutorial, its practicality will come home and you definitely will love how much understanding you will get.

#### Collision Domain

A collision domain is a section of a network where data packets can collide with one another when being sent on a shared medium or through repeaters, in particular, when using early versions of Ethernet. A network collision occurs when more than one device attempts to send a packet on a network segment at the same time. Collisions are resolved using carrier sense multiple access with collision detection in which the competing packets are discarded and re-sent one at a time. This becomes a source of inefficiency in the network. Please see the Animation1.1.
Now that we have a fair idea of these two basic terms in networking, lets move on to the specific funtions of the primary segmenting networking devices

### Switch

Well, not the wall switch that turns on the light in our room. This is a networking device and that is its name. One thing we all should note about Switches are that, they are not used to create internetworks, i.e., they do not break up broadcast domains by default. Instead, they are employed in a network to add functionality to a network LAN. Its basic purpose is to make Local Area connections work better - to optimize performance and - providing more bandwidth for the LAN's users.
Another fascinating feature of Switches are that, they don't forward packets to other networks as routers do. Instead they only switch frames from one port to another within switched networks. Okay, you may be thinking, "What are frames?" I will tell you more about them in this chapter. I promise!
You need to know too that the difference between a switch and its brother bridge are just a few. Switches are like Multiport bridges. Whereas a switch will have about 24 ports on it, a bridge will have only a maximum of 4.
The capabilities of switches are very many. In fact, they serve real purpose in a network and serve a purpose that would be much difficult to be well handled by either a hub or its elder brother, the Router.

### Router

A router is a device that forwards data packets between computer networks, creating an overlay internetwork. A router is connected to two or more data lines from different networks. When a data packet comes in on one of the lines, the router reads the address information in the packet to determine its ultimate destination. Then, using information in its routing table or routing policy, it directs the packet to the next network on its journey. Routers perform the "traffic directing" functions on the Internet. A data packet is typically forwarded from one router to another through the networks that constitute the internetwork until it gets to its destination node.
Routers perform an essential part in computing networks by breaking up broadcast domains. This is important because when a host or server sends a network broadcast, every device on the network must read and process that broadcast. However, when the interface of the router receives such broadcast, it can respond by basically saying, "Thanks but no thanks!" and will just discard the broadcast without forwarding to other networks.
There are two basica advantages of using a router and they are:
   * They do not forward broadcast by default.
   * They can filter the network based on Layer 3, i.e, the Network Layer information (e.g., IP address)
Furthermore, routers kind of achieve the same functionalily in a network such as listed below:
   * Packet switching
   * Packet filtering
   * Internetwork Communication
   * Data path selection
One thing we should note too is that, routers are in a way, switches - yes, Layer 3 Switches. We will get to understand more of the Layers in this tutorial. Just hold on tight.
As an administrator, you might be wondering getting switches to replace your hubs in the network. Well, it is recommended you do so since the switches will truly and really sensibly segment your network. Talking about efficiency, you will have it. Speed wouldn't be left out either. The best way to get traffic passing through your network with absolutely less or no high latency or increased overhead is to employ the use of switches instead of hubs. So, my dear friend, get your hubs replaced with switches and your enterprise network connectivity will never be the same. By getting routers included in the networking architecture of your enterprise will really pay off.

## Internetworking Model

In the beginning of computer networks, only computers from the same manufacturer could communicate. So a device from company A when put in a network with devices from company B will just be a mess and no communication could be established. So a typical example is that, companies either run the DECnet solution or the IBM solution. Mixing them will be the starting for mixed confusion between the devices.
However, in the late 1970, the Open System Interconnection (OSI) reference model was introduced. It was introduced by the International Organization for Standardization (ISO). The basic aim of this standardization was to help vendors create interoperable network devices and software in the form of protocols so tat different vendor networks could work with each other. Like world peace. it will perhaps never happen completely, but its still a great goal.
This OSI model is basically the primary architecture model for networks since it describes how information are communicated from an application on one computer through the network media to an application on another computer. And by breaking this communication process down, its is known as layers.

### The Layered Approach

A reference model is a conceptual blueprint of how communications should take place. It addresses all the processes required for effective communication and divides these processes into logical groupings called layers. When a communication system is designed in this manner, it’s known as layered architecture.
Every section or layer in the layered approach system has specific function(s) they perform distinctively of which the upper layer needs to function properly. It would be obviously ridiculous for the Application Layer to function correctly if the Network layer isn't in place or is functioning badly. Thus, the combined orchestration of these layers output better results and expectations are met maximumly.
The Layered approach really helps. Its helps software developers concentrate on building their applications and softwares without the worry of how their softwares are gonna communicate over the internet or how their software are gonna share data with other devices. Other than that, each programmer or software developer would have to program a distinct communication mechanism involving protocols and anything that will make the data communication between his device and other devices on the networ work. However, it isn't the case since developers are concerned with their softwares, and the layered approach does the rest. But, note that, the software developer willl have to make sure his software conforms to the OSI model which is an international standard. Otherwise, his software will only be the software on earth that isn't able to communicate with other devices.