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Networking Basics

Network switch explained

Just like hubs and bridges, a switch is used to connect multiple hosts together, but it has many advantages over them. Switch is an OSI Layer 2 device, which means that it can inspect received traffic and make forwarding decisions. Each port on a switch is a separate collision domain and can run in a full duplex mode (photo credit: Wikipedia).

Cisco switch

A switch manages the flow of data across a network by inspecting the incoming frame’s destination MAC address and forwarding the frame only to the host for which the data was intended. Each switch has a dynamic table (called the MAC address table) that maps MAC addresses to ports. With this information, a switch can identify which system is sitting on which port and where to send the received frame.

To better understand how a switch works, consider the following example:

Network switch explanation

As you can see from teh example above, Host A is trying to communicate with Host C and sends a packet with the Host C’s destination MAC address. The packet arrives at the switch, which looks at the destination MAC address. The switch then searches that MAC address in its MAC address table. If the MAC address is found, the switch then forwards the packet only out the port connected to the frame’s destination. Hosts connected to other ports will not receive the frame.

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Networking Basics

Network bridge explained

A network bridge is a device that divides a network into segments. Each segment represent a separate collision domain, so the number of collisions on the network is reduced. Also, because each collision domain has its own separate bandwidth, a bridge also improves the overall network performance.

NOTE
Unlike hubs, bridges allow multiple devices to send at the same time. This is why there are considered to be predecessors of network switches.

 

A bridge works at the Data link layer (Layer 2) of the OSI model, just like a switch does. It inspects incoming traffic and decide whether to forward it or filter it. Each incoming Ethernet frame is inspected for destination MAC address. If the bridge determines that the destination host is on another segment of the network, it forwards the frame to that segment.

Consider the following network:

Network bridges explained

In the example above we have a network of four computers. The network is divided into segments by a bridge. Each segment is a separate collision domain with its own bandwidth. Let’s say that Host A wants to communicate with Host C. Host A will send the frame with the Host C’s destination MAC address to the bridge. The bridge will inspect the frame and forward it to the segment of the network Host C is on.

Network bridges offered substantial improvements over network hubs, but are not widely used anymore in modern LANs – switches are commonly used instead. Here is why:

  • most bridges have only 2 or 4 ports. A switch can have tens or even hundreds of ports
  • bridges are software based, while switches are hardware-based and use chips (ASICs) when making forwarding decisions, which makes them much faster than bridges
  • switches can have multiple spanning-tree instances, bridges can have only one
  • switches can have multiple broadcast domains (one per VLAN)

 

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Networking Basics

OSI & TCP/IP models

OSI model

OSI (Open Systems Interconnection) model was created by the International Organization for Standardization (ISO), an international standard-setting body. It was designed to be a reference model for describing the functions of a communication system. The OSI model provides a framework for creating and implementing networking standards and devices and describes how network applications on different computers can communicate through the network media.

The OSI model has seven layers, with each layer describing a different function of data traveling through a network. Here is the graphical representation of these layers:

OSI model
The layers are usually numbered from the last one, meaning that the Physical layer is considered to be the first layer. It is useful to remember these layers, since there will certainly be a couple of questions on the CCNA exam regarding them. Most people learn the mnemonic „Please Do Not Throw Sausage Pizza Away“:

OSI mnemonic
So, what is the purpose of these layers?
They are most commonly used by vendors. They enable them to implement some functionality into a networking device, which then enables easier interoperability with devices from other vendors.

Here is a brief description of each of the layers of the OSI model.

  • Physical – defines how to move bits from one device to another. It details how cables, connectors and network interface cards are supposed to work and how to send and receive bits.
  • Data Link – encapsulates a packet in a frame. A frame contains a header and a trailer that enable devices to communicate. A header (most commonly) contains a source and destination MAC address. A trailer contains the Frame Check Sequence field, which is used to detect transmission errors. The data link layer has two sublayers:

1. Logical Link Control – used for flow control and error detection.
2. Media Access Control – used for hardware addressing and for controlling the access method.

  • Network – defines device addressing, routing, and path determination. Device (logical) addressing is used to identify a host on a network (e.g. by its IP address).
  • Transport – segments big chunks of data received from the upper layer protocols. Establishes and terminates connections between two computers. Used for flow control and data recovery.
  • Session – defines how to establish and terminate a session between the two systems.
  • Presentation – defines data formats. Compression and encryption are defined at this layer.
  • Application – this layer is the closest to the user. It enables network applications to communicate with other network applications.

It is a common practice to reference a protocol by the layer number or layer name. For example, HTTPS is referred to as an application (or Layer 7) protocol. Network devices are also sometimes described according to the OSI layer on which they operate – e.g. a Layer 2 switch or a Layer 7 firewall.

The following table shows which protocols reside on which layer of the OSI model:

OSI protocols

TCP/IP model

The TCP/IP model was created in the 1970s by the Defense Advance Research Project Agency (DARPA) as an open, vendor-neutral, public networking model. Just like the OSI model, it describes general guidelines for designing and implementing computer protocols. It consists of four layers: Network Access, Internet, Transport, and Application:

TCP IP model

The following picture show the comparison between the TCP/IP model and OSI model:

TCP IP and OSI model comparison

As you can see from the picture above, the TCP/IP model has fewer layers than the OSI model. The Application, Presentation, and Session layers of the OSI model are merged into a single layer in the TCP/IP model. Also, Physical and Data Link layers are called Network Access layer in the TCP/IP model. Here is a brief description of each layer:

  • Link – defines the protocols and hardware required to deliver data across a physical network.
  • Internet – defines the protocols for the logical transmission of packets over the network.
  • Transport – defines protocols for setting up the level of transmission service for applications. This layer is responsible for reliable transmission of data and the the error-free delivery of packets.
  • Application – defines protocols for node-to-node application communication and provide services to the application software running on a computer.

Differences between OSI and TCP/IP model

There are some other differences between these two models, besides the obvious difference in the number of layers. OSI model prescribes the steps needed to transfer data over a network and it is very specific in it, defining which protocol is used at each layer and how. The TCP/IP model is not that specific. It can be said that the OSI model prescribes and TCP/IP model describes.

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Networking Basics

Network hubs explained

A hub serves as a central point to which all of the hosts in a network connect to. It is an OSI Layer 1 device and has no concept of Ethernet frames or addressing – it simply receives the signal from one port and sends it out to all other ports. Here is an example 4-port Ethernet hub (image source: Wikipedia):

Network hub

As mentioned above, hubs have no way of distinguishing out which port a signal should be sent to; instead, an electrical signal is sent out each port. All nodes on the network will receive data, and the data will eventually reach the correct destination, but with a lot of unnecessary network traffic:

How hubs work

In the example above you can see that the hub has sent out the receiving signal out all other ports, except the incoming port. Hubs are therefore considered obsolete and switches are commonly used instead in modern LANs. Hubs have numerous disadvantages over switches, such as:

  • they are not aware of the traffic that passes through them
  • they create only one large collision domain
  • a hub typically operates in half duplex
  • there is also a security issue with hubs since the traffic is forwarded to all ports (except the source port), which makes it possible to capture all traffic on a network with a network sniffer!

 

NOTE
Hubs are also known as multiport repeaters because that is basically what they do – repeat the electrical signal that comes in one port out all other ports (except the incoming port).

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Networking Basics

What is a network?

A computer network can be described as a system of interconnected devices that can communicate using some common standards (called protocols). These devices communicate to exchange resources (e.g. files and printers) and services.

Here is an example network consisting of two computers connected together:

network of two computers only

In the example above, the two computers are directly connected using a cable. This small network can be used to exchange data between just these two computers.

What if we want to expand our network? Then we can use a network device, either a switch or a hub, to connect more than two computers together:

a network with a hub

Now all of the devices on the network can communicate with each other.

We’ll talk more about hubs and switches in just a moment. For now, just remember that these devices serve as a central point to which all of the computers connect to.

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