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ICND1 break down -- What is A Network

You might wonder what is a network? 

If you are looking at this post, you are on a network called internet right now. 

Look around, is your computer/desktop/laptop siting around a place like this? 

That box with antenna is a wireless router. That blackbox connecting to your cable TV Cable is a cable modem. Most likely, your PC is connecting to your router with an ethernet cable, your router then connects to the cable modem using another ethernet cable, finally the cable modem connects to a cable TV outlet in the wall using a coaxial cable, which is usually known as TV cable. The coaxial cable connects to your internet service provider's network. The internet service provider is the company provides your internet access and most likely TV access as well.

Wait, what if you are reading this post using an ipad, iphone or android? How your mobile devices get internet connection? There are many possibilities, the following two are most common.

One possibility is your mobile device is connecting to that wireless router with wifi. Wifi is a microwave generated by your wireless router and mobile devices, the microwave carries signal so that your device and router are constantly communicating. The content of this post are pass down to your wireless router via TV cable and ethernet cable, then pass to your device via wifi.

Another possibility is your device is connecting to a carrier network like AT & T, verizon, sprint or T mobile. The carrier network also use microwave as the media to pass signal. However, compare to your wireless router's microwave, this type of microwave have shorter wavelength, so it can travel a much longer distance. Long enough to span from your cellphone to the carriers' tower somewhere on the street or on the top of a hill. 

Well, something you might not know, your cellphone's battery draws faster when you upload images comparing to when you download images. The reason is, when uploading, your phone generate  microwave signal for the tower to receive. Surprise, hah?

Anyway, that's the network we saw with the eyes. Under the hood, tons of things are happening. Here is the network we can only see with our imagination.

How internet works
How TCP/IP works

A little bit of history.  The Advanced Research Projects Agency (ARPA) designed "Advanced Research Projects Agency Network" (ARPANET) for the United States Department of Defense. It was the first computer network in the world in late 1960s and early 1970s. With the help of universities and volunteers, this DoD origin network develops into today's open network based on TCP/IP protocol. There are a few proprietary networks developed by IBM, DEC in the history, however TCP/IP dominates today's computer network.

Again, what is a network?

By definition, a computer network is a connected collection of devices and end systems, such as computers and servers, that can communicate with each other. A computer network have four major categories of interconnected hardware devices.

  • Personal Computers (PCS) : The PCs include devices act as endpoints devices in the network, such as laptop, desktop, file servers, mobile phones.

  • Interconnections: The interconnections consist of components that provide a means for data to travel from one point to another point in the network, such as Network interface cards (NICs), Network media (cables or wireless media) and connectors.

  • Switches: Switches are devices that provide network attachment to the end systems and intelligent switching of the data within the local network.

  • Routers: Routers interconnect networks and choose the best paths between networks.

The main functions of computer networks is to enable end users to share both information and hardware resources. The major resources that are shared in a computer network include data and applications, physical resources, network storage and backup devices.

There are many applications that are aware of network communication mechanisms, the most common network applications are:

  • Email: Examples include gmail, Microsoft Outlook.

  • Web browser: Examples include Chrome, Microsoft Internet Explore, Safari and Firefox.

  • Instant messaging: Examples include slack, facebook Messager, WhatsApp.

  • Collaboration: Collaboration allows individuals or groups on a network to work together on a project. Examples include Lotus Notes and wiki.

  • Database: Database enables users on a network to store information in central data servers so that others on the network can easily retrieve them. Examples include NoSQL, MySQL, Oracle and Microsoft SQL Server.

Networks can be described and compared according to network performance and structure, as follows:

  • Speed: Speed is a measure of how fast data is transmitted over the network. People are often concerned about measuring the maximum data throughput rate or Bandwidth of a communications link or network access. A typical method of performing a measurement is to transfer a 'large' file and measure the time taken to do so. The throughput is then calculated by dividing the file size by the time. The throughput of communications links is measured in bits per second (bit/s or bps), kilobits per second (kbit/s or kbps), megabits per second (Mbit/s or mbps) and gigabits per second (Gbit/s or gbps). Here kilo, mega and giga are the standard S.I. prefixes indicating multiplication by 1,000 (kilo), 1,000,000 (mega), and 1,000,000,000 (giga). The throughput of a network is determined by factors such as network topology and number of users on the network.

  • Cost: The general cost of components, installation, and maintaenance of the network.

  • Security: Security indicates how secure the network is, including the data that is transmitted over the network.

  • Availability: A measure of the probability that the network will be available for use when required.

  • Scalability: Scalability is a measure of how well the network can accomodate more users and data transmission requirements.

  • Reliability: Reliability indicates how dependable the network components are.

  • Topology: Either physical topology, which is the arrangement of the cable, network devices and end systems, or the logical topology, which is the path that the data signals take through the physical topology.

Internet connectivity is a must in today's network. The most common methods to connect the small office to the Internet includes:

  • Digital Subscriber Line (DSL): DSL use the existing telephone lines as the data signal carrying media.

  • Cable: Cable uses the cable television (CATV) as communication infrastructure.

  • Serial: Serial uses the classic digital local loops.

We will talk about these WAN connection types in detail in later lessons.

CCENT will test fundamentals of switches and routed internetworks. You must thoroughly understand the basic concepts of computer networks.
ICND1 and ICND2 break down


  1. Nice website, bookmarked for future reference.

  2. thanks, this is a very useful blog.