Computer Networking for beginner Ethical Hackers

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What is a network?

A network is a set of devices that are connected with a physical media link. In a network, two or more nodes are connected by a physical link, or two or more networks are connected by one or more nodes. A network is a collection of devices connected to each other to allow the sharing of data.

What is Network topology?

Network topology specifies the layout of a computer network. It shows how devices and cables are connected to each other.

Types of Network Topology :


  1. Star topology is a network topology in which all the nodes are connected
    to a single device known as a central device.
  2. Star topology requires more cable compared to other topologies.
    Therefore, it is more robust as a failure in one cable will only disconnect a
    specific computer connected to this cable.
  3. If the central device is damaged, then the whole network fails.
  4. Star topology is very easy to install, manage and troubleshoot. It is
    commonly used in office and home networks.


  1. Ring topology is a network topology in which nodes are exactly connected to two
    or more nodes and thus, forming a single continuous path for the transmission.
  2. It does not need any central server to control the connectivity among the nodes.
  3. If the single node is damaged, then the whole network fails.
  4. Ring topology is very rarely used as it is expensive, difficult to install and
  5. Examples of Ring topology are SONET network, SDH network, etc.

3)-Bus :

  1. Bus topology is a network topology in which all the nodes are connected to a
    single cable known as a central cable or bus.
  2. It acts as a shared communication medium, i.e., if any device wants to send the
    data to other devices, then it will send the data over the bus which in turn sends
    the data to all the attached devices.
  3. Bus topology is useful for a small number of devices.
  4. As if the bus is damaged then the whole network fails.

4)- Mesh :

  1. Mesh topology is a network topology in which all the nodes are individually
    connected to other nodes.
  2. It does not need any central switch or hub to control the connectivity among the
  3. Mesh topology is categorized into two parts: Fully connected mesh topology:
    In this topology, all the nodes are connected to each other. Partially connected
    mesh topology: In this topology, all the nodes are not connected to each other.
  4. It is robust as a failure in one cable will only disconnect the specified computer
    connected to this cable.
  5. Mesh topology is rarely used as installation and configuration are difficult when
    connectivity gets more.
  6. Cabling cost is high as it requires bulk wiring.

5)-Tree :

  1. Tree topology is a combination of star and bus topology. It is also known as the
    expanded star topology.
  2. In tree topology, all the star networks are connected to a single bus.
  3. Ethernet protocol is used in this topology.
  4. In this, the whole network is divided into segments known as star networks which
    can be easily maintained. If one segment is damaged, there is no effect on other
  5. Tree topology depends on the “main bus,” and if it breaks, then the whole
    network gets damaged

6)-Hybrid :

  1. A hybrid topology is a combination of different topologies to form a resulting
  2. If star topology is connected with another star topology, then it remains a star
    topology. If star topology is connected with different topology, then it becomes a
    Hybrid topology.
  3. It provides flexibility as it can be implemented in a different network environment.

Different types of Networks:

Networks can be divided on the basis of area of distribution. For example:

● PAN (Personal Area Network): Its range limit is up to 10 meters. It is created for
personal use. Generally, personal devices are connected to this network. For example
computers, telephones, fax, printers, etc.
● LAN (Local Area Network): It is used for a small geographical location like office,
hospital, school, etc.
● HAN (House Area Network): It is actually a LAN that is used within a house and used to
connect homely devices like personal computers, phones, printers, etc.
● CAN (Campus Area Network): It is a connection of devices within a campus area which
links to other departments of the organization within the same campus.
● MAN (Metropolitan Area Network): It is used to connect the devices which span to
large cities like metropolitan cities over a wide geographical area.
● WAN (Wide Area Network): It is used over a wide geographical location that may range
to connect cities and countries.
● GAN (Global Area Network): It uses satellites to connect devices over the global area.

VPN (Virtual Private Network) :

VPN or the Virtual Private Network is a private WAN (Wide Area Network) built on the internet. It allows the creation of a secured tunnel (protected network) between different networks using the internet (public network). By using the VPN, a client can connect to the organization’s network remotely.

Advantages of VPN :

  1. VPN is used to connect offices in different geographical locations remotely and is
    cheaper when compared to WAN connections.
  2. VPN is used for secure transactions and confidential data transfer between
    multiple offices located in different geographical locations.
  3. VPN keeps an organization’s information secured against any potential threats or
    intrusions by using virtualization.
  4. VPN encrypts the internet traffic and disguises the online identity.
  • Types of VPN:

● Access VPN: Access VPN is used to provide connectivity to remote mobile users and
telecommuters. It serves as an alternative to dial-up connections or ISDN (Integrated
Services Digital Network) connections. It is a low-cost solution and provides a wide
range of connectivity.
● Site-to-Site VPN: A Site-to-Site or Router-to-Router VPN is commonly used in large
companies having branches in different locations to connect the network of one office to
another in different locations. There are 2 sub-categories as mentioned below:
● Intranet VPN: Intranet VPN is useful for connecting remote offices in different
geographical locations using shared infrastructure (internet connectivity and servers)
with the same accessibility policies as a private WAN (wide area network).
● Extranet VPN: Extranet VPN uses shared infrastructure over an intranet, suppliers,
customers, partners, and other entities and connects them using dedicated connections.


An IP address is a 32-bit dynamic address of a node in the network. An IPv4 address has 4 octets of 8-bit each with each number with a value up to 255. IPv4 classes are differentiated based on the number of hosts it supports on the network.
There are five types of IPv4 classes and are based on the first octet of IP addresses
which are classified as Class A, B, C, D, or E.

OSI (Open System Interconnections) -Most Imp

It is a network architecture model based on the ISO standards. It is called the OSI model as it deals with connecting the systems that are open for communication with other systems. The OSI model has seven layers.
The principles used to arrive at the seven layers can be summarized briefly as below:

  1. Create a new layer if a different abstraction is needed.
  2. Each layer should have a well-defined function.
  3. The function of each layer is chosen based on internationally standardized

Seven Layers :

  1. Physical Layer
    ● It is the lowest layer of the OSI reference model.
    ● It is used for the transmission of an unstructured raw bit stream over a physical
    ● Physical layer transmits the data either in the form of electrical/optical or
    mechanical form.
    ● The physical layer is mainly used for the physical connection between the
    devices, and such physical connection can be made by using twisted-pair cable,
    fibre-optic or wireless transmission media.
  2. DataLink Layer
    ● It is used for transferring the data from one node to another node.
    ● It receives the data from the network layer and converts the data into data frames
    and then attaches the physical address to these frames which are sent to the
    physical layer.
    ● It enables the error-free transfer of data from one node to another node.
  1. Network Layer
    ● Network layer converts the logical address into the physical address.
    ● The routing concept means it determines the best route for the packet to travel from
    source to the destination.
    Functions of network layer :
    ● Routing: The network layer determines the best route from source to destination. This
    function is known as routing.
    ● Logical addressing: The network layer defines the addressing scheme to identify each
    device uniquely.
    ● Packetizing: The network layer receives the data from the upper layer and converts the
    data into packets. This process is known as packetizing.
    ● Internetworking: The network layer provides the logical connection between the
    different types of networks for forming a bigger network.
    ● Fragmentation: It is a process of dividing the packets into fragments..
  1. Transport Layer
    ● It delivers the message through the network and provides error checking so that no error
    occurs during the transfer of data.
    ● It provides two kinds of services:
    ○ Connection-oriented transmission: In this transmission, the receiver sends the
    acknowledgement to the sender after the packet has been received.
    ○ Connectionless transmission: In this transmission, the receiver does not send
    the acknowledgement to the sender.
  1. Session Layer
    ● The main responsibility of the session layer is beginning, maintaining and ending the
    communication between the devices.
    ● Session layer also reports the error coming from the upper layers.
    ● Session layer establishes and maintains the session between the two users.
  2. Presentation Layer
    ● The presentation layer is also known as a Translation layer as it translates the data from
    one format to another format.
    ● At the sender side, this layer translates the data format used by the application layer to
    the common format and at the receiver side, this layer translates the common format into
    a format used by the application layer.
  1. Application Layer
    ● Application layer enables the user to access the network.
    ● It is the topmost layer of the OSI reference model.
    ● Application layer protocols are file transfer protocol, simple mail transfer protocol,
    domain name system, etc.
    ● The most widely used application protocol is HTTP(Hypertext transfer protocol ). A user
    sends the request for the web page using HTTP.

TCP/IP Reference Model :

It is a compressed version of the OSI model with only 4
layers. It was developed by the US Department of Defence (DoD) in the 1860s. The
name of this model is based on 2 standard protocols used i.e. TCP (Transmission
Control Protocol) and IP (Internet Protocol).

  1. Link : Decides which links such as serial lines or classic Ethernet must be used
    to meet the needs of the connectionless internet layer. Ex – Sonet, Ethernet
  2. Internet : The internet layer is the most important layer which holds the whole
    architecture together. It delivers the IP packets where they are supposed to be
    delivered. Ex – IP, ICMP.
  3. Transport : Its functionality is almost the same as the OSI transport layer. It
    enables peer entities on the network to carry on a conversation. Ex – TCP, UDP
    (User Datagram Protocol)
  4. Application : It contains all the higher-level protocols. Ex – HTTP, SMTP, RTP,


HTTP is the HyperText Transfer Protocol which defines the set of rules and standards
on how the information can be transmitted on the World Wide Web (WWW). It helps the
web browsers and web servers for communication. It is a ‘stateless protocol’ where each
command is independent with respect to the previous command. HTTP is an
application layer protocol built upon the TCP. It uses port 80 by default.
HTTPS is the HyperText Transfer Protocol Secure or Secure HTTP. It is an advanced
and secured version of HTTP. On top of HTTP, SSL/TLS protocol is used to provide
security. It enables secure transactions by encrypting the communication and also
helps identify network servers securely. It uses port 443 by default.

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