Beginning of my free CCNA training series
It is important in any IT job to understand the basics of computer networking. Computer networking is a part of everything in IT, and all of the more complex technology is built from this foundation. Many issues can be solved just by knowing how computer networking functions. This article will cover the basics of how computers interact with each other, including hubs, switches and routers. This is not an all-inclusive article and is meant to be a basic introduction to how networks functions. I also covered some basic networking terms here. I intend this page to be the beginning of free CCNA training for all to get started with.
In order to understand how network devices work, you have to understand the basic concepts. Computer networking functions much like a postal service. The data is like letters which are encapsulated and transit wires which are like highways and roads. The highways lead to side roads which lead to addresses. Every device has a physical address, called a Media Access Control address (MAC). They also have an Internet Protocol (IP) address, and sub-net mask. I will cover sub-netting in a separate article because it is a beast all on its own. Your home address is always 127.0.0.1 (or localhost).
Open System Interconnection Model
The OSI model splits computer networking into 7 layers. Layer 1 is called the Physical layer, and mostly contains protocols dealing with the 1’s and 0’s crossing various types of wires. Hubs operate on the physical layer. I will cover this in a separate article in the future. Layer 2 is called the Data-Link layer, and is where MAC addresses and ARP tables are. The 3rd layer is called the Network layer and is where routers are. Layer 4 is the Transport layer, where TCP and UDP are located. The 5th layer is the Session layer, which basically connects one application to another. Layer 6 is called the Presentation layer, and is where data from layer 7 is converted into a network-ready format, like encryption. The final layer is layer 7, the Application layer and is where all the end-user processes are.
In the beginning hubs were used to connect machines in a network. Hubs are less complex devices that simply broadcast every piece of data to every port on the hub. This is bad because it is very inefficient and another way had to be figured out. Computer networking uses a similar concept as a postal service does. Locations are setup with addresses that can be saved in a table, and the post office needs to maintain the table so that they can send you a letter addressed to you.
In order to organize a system for transferring data, each machine has to have an address, and a table of these addresses must be stored for reference. The first part of that is to give it an address that never changes, and in computing that is called the MAC address. It is embedded in every device physically and will always be the same. The second part is to keep information about where all of the addresses are located. Something has to maintain a list of them, and that device is called a switch. They use what is called the Address Resolution Protocol to keep a table of the MAC addresses that are connected to it.
When a machine is connected to a switch, the switch probes it to get its MAC address. The switch saves this information in something called an Address Resolution Protocol table. If machine A and machine B are connected to the same switch, it knows the MAC addresses and sends the information to the right source. Switches maintain this list of where to send data and update it whenever new information is learned.
If machine C is added to the switch, and sends a message to machine D which is connected through the network, the switch will broadcast the message to all of its ports much like the hub. The difference is that when it receives a response from its network connected, it will remember that is where machine D responded from. When the response comes from machine D, the switch will broadcast it to machine A, B and C just like a hub. The difference on a switch, once again, is when the switch learns where machine C is, it saves that information in its table.
Switches are great devices for a small office, but what if you need to connect more devices to each other, or devices from another location like machine D in the example above? In our postal example, what happens when you need to send a letter from one country to another? This is where routers come into play. Routers enable communication between different networks. They store IP tables that contain information about how to get from one network to another and send traffic efficiently. They are smart devices and are capable of building routes on their own, or using pre-programmed routes.
With our example above, if machine D is connected to a switch in another room, that switch would be connected to a router. Machine D sends its data to the switch, which broadcasts it on all of its ports. The router gets the data and knows to send the packet to the first switch. Switches and routers are capable of sharing information in order to build routing tables. Routers don’t know the MAC addresses of devices and switches don’t know the IP address of devices.
Free CCNA training
To recap, hubs are less complex devices that broadcast to everything connected to it. Switches are smarter, broadcast once until they learn your MAC, but are still limited to only their network. Routers are the smartest devices, maintain a routing table of IP addresses and connect one network to another. There are unique devices such as layer 3 switches which function like normal switches, but are capable of limited routing as well. I will dive deeper into computer networking over the coming weeks in an effort to provide free CCNA training for people to study when they get started.
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