Computer Networking is the practice of connecting computers together to enable communication and data exchange between them.

Basics building blocks of a Computer network are Nodes and Links. Each device/node has an IP Address, that helps in identifying a device.

Network

A collection of computers, servers, mainframes, network devices, and other devices connected to one another for sharing data and resources.

Components

1) Nodes: These are devices that are connected to a network. These can include computers, Servers, Printers, Routers, Switches, and other devices.

2) Links:

These are wires that connect the nodes together.

Types of Networks

Based on size:

Personal Area Network (PAN):

It is used for connecting personal devices like smartphones, tablets, and laptops.

Ex. Bluetooth connections between a smartphone and a wireless headset.

Local Area Network (LAN):

It is used for connecting computers and devices within an office or home.

Ex. An office/college network connecting all the computers and printers in a building.

Wide Area Network (WAN):

It covers a large geographic area, often spanning cities, countries, or even continents.

Ex. ISPs use WAN technologies to provide internet connectivity to homes, businesses, and institutions.

Metropolitan Area Network (MAN):

It covers a city or a large campus. It provides high-speed network access to a city or large organization.

Ex. A city-wide Wi-Fi network or a university network spanning multiple buildings across a campus.

Global Area Network (GAN):

It spans across the entire globe.

Ex. The global internet infrastructure.

Based on Architecture:

Client-Server Network:

It is centralized, with one or more servers providing resources and services to client devices.

Ex. A company network where a file server provides access to documents and applications to client computers.

Peer-to-Peer (P2P) Network:

It is decentralized, with each device acting as both a client and a server.

Ex. Home networks where computers share files directly with each other.

Based on Communication Method:

Wired Network:

It uses physical cables (e.g., Ethernet) for communication.

Example: A traditional office LAN connected via Ethernet cables.

Wireless Network:

It uses radio waves or infrared signals for communication.

Example: Wi-Fi networks that connect devices without physical cables.

Based on Functionality:

Storage Area Network (SAN):

Function: Provides high-speed access to consolidated data storage.

Use Case: Common in data centers and large organizations for data storage and retrieval.

Example: A network connecting servers to storage devices for high-performance data access.

Virtual Private Network (VPN):

It is a technology that enhances the security and privacy of your internet connection by creating a private, encrypted tunnel through which our data travels.

How VPNs work?

1) When you connect to a VPN, it encrypts your data. This means that the information sent from your device is scrambled into a code that is unreadable to anyone who intercepts it. This encryption makes it much harder for hackers, ISPs, or any other entities to see what you're doing online.

2) VPNs create a secure "tunnel" between your device and the VPN server. This tunnel encapsulates your data and the data you send travels through this tunnel to the VPN server, which then decrypts it and forwards it to its final destination

3) When you use a VPN, your internet traffic is routed through the VPN server

A VPN client is software or an application that allows users to connect to a VPN server and access the VPN service.

Ex. NordVPN, ExpressVPN, OpenVPN

Content Delivery Network (CDN):

It distributes content across multiple servers to improve delivery speed and reliability. It enhances the performance of websites and online services by caching content closer to users.

Ex. Services like Akamai or Cloudflare that cache and deliver web content.

Based on topology:

It is a physical layout of the network, connecting the different nodes using the links.

Mesh:

In a mesh topology, each and every device should have a dedicated point-to-point connection with each and every other device in the network. It is robust as failure in one link only disconnects that node. It is rarely used, and installation and management are difficult.

Star:

All the nodes are connected to one single node known as the central node. It is easy to install and reconnect as compared to Mesh Topology. Star Topology doesn’t have Fault Tolerance Technique. It is used in home and office networks.

Bus:

All devices are connected through a single cable

If the main cable gets damaged, it will damage the whole network.

Ring:

Each device is connected with repeaters in a circle-like ring.

It is used very rarely as it is expensive and hard to install and manage.

Types of DNS:

1) Authoritative

These servers store the DNS records for domain names and respond with authoritative answers to queries about those domains. Managed by organizations, businesses, and domain registrars like Godaddy, Namecheap etc.

2) Recursive

These servers perform the complete DNS lookup process on behalf of clients. They start from the root DNS servers and query other DNS servers until they resolve the requested domain name.

Managed by ISPs (such as Comcast, AT&T etc) and public DNS providers (such as Google Public DNS (8.8.8.8), Cloudflare DNS (1.1.1.1))

3) Root

Managed by ICANN (The Internet Corporation for Assigned Names and Numbers) and root server operators (such as Verisign)

Types of DNS Records

1) A Record (Address Record)

Maps a domain name to an IP address (IPv4).

2) CNAME Record (Canonical Name Record)

It points one domain name to another domain name (an alias).

3) MX Record (Mail Exchange Record)

It specifies the mail servers responsible for receiving email on behalf of a domain.

4) NS Record (Nameserver Record)

It specifies the nameservers for a domain, i.e., the servers that are authoritative for that domain's DNS records.

5) TXT Record (Text Record)

It text-based information, often for email security or verification.

Networking Devices (Layer 3)

Router

It is a networking device that connects multiple networks and routes data packets between them. It forwards data packets between computer networks. It determines the best path for data to travel from the source to the destination. They use routing tables and protocols to make decisions about where to send packets.

Routers maintain routing tables that store information about the network paths and the best routes to various destinations. These tables are updated dynamically using routing protocols (e.g., OSPF, BGP, EIGRP).

It performs NAT(Network Address Translation),

(It takes the Internet from the modem and transfers it to your wireless devices)

Network Interface Cards/Network Adapter (NIC)

They are hardware components that allow a computer or other device to connect to a network. Each NIC has a unique MAC (Media Access Control) address, which is a hardware identifier assigned to the NIC.

Monitor mode is a special mode for wireless network adapters that allows them to capture all wireless traffic in the vicinity, not just traffic intended for their own network interface

Note: In a virtual machine, the network adapters presented to the guest operating system (like Linux) are controlled by the hypervisor (such as VirtualBox, VMware, or Hyper-V). The VM might only show the virtual Ethernet adapter and not the host's Wi-Fi adapter.

Access Points (APs)

It provides connectivity to wireless devices in a network. They operate at both the Data Link Layer (for managing MAC addresses) and the Physical Layer (for radio frequency communication).

Modem (modulator-demodulator)

It is a device that enables computers and other digital devices to communicate over telephone lines, cable systems, or other types of communication media by converting digital data into analog signals and vice versa. Its primary function is to enable internet connectivity by converting digital data from a computer into a format suitable for transmission over various types of communication lines and vice versa.

(Modem brings the requested information from the internet to your network)

Note: Many modern internet service providers (ISPs) offer modem-router combos, which integrate both the modem and router functionalities into a single device

Unique Identifiers of Network

Hostname

Each device in the network is associated with a unique device name.

IP / Logical address

To identify each device in the world-wide-web, the Internet Assigned Numbers Authority (IANA) assigns an IP address as a unique identifier to each device on the Internet.

whereismyipaddress

Private IP Address:

Inside your LAN, your router assigns private IP addresses to each device. These addresses are only valid within your local network.

2)IPv6

IPv6 was designed by the Internet Engineering Task Force (IETF) in December 1998 with the purpose of superseding IPv4 due to the global exponentially growing internet of users.

3001:0da8:75a3:0000:0000:8a2e:0370:7334

340 undecillion unique address space.

128-bit address space

MAC / Physical Address (Media Access Control)

It is a unique identifier assigned to a network interface controller (NIC) for communications on the physical network segment such as Wi-Fi, Ethernet and Bluetooth.

It is typically represented as a string of six groups of two hexadecimal digits, separated by colons (:) or hyphens (-).

Ex. 00:1A:2B:3C:4D:5E

It is a 48-bit (6 bytes) address. It is used at the data link layer of the OSI model to ensure that data is directed to the correct device within a local network, such as a LAN

VOIP (Voice over Internet Protocol)

It is a technology that enables voice communication over the internet or other IP-based networks instead of traditional telephone lines. It converts voice signals into digital data packets and transmits them over IP networks. The data is then converted back into voice signals at the receiving end. VoIP services often come with features like call forwarding, voicemail, video calling, and conferencing

Address Resolution Protocol (ARP)

It is used to map IP addresses to MAC (Media Access Control) addresses within a local network. When a device wants to communicate with another device on the same local network, it uses ARP to determine the MAC address associated with the destination IP address.

This is maintained by all network devices.

OSI (Please Do Not Throw Sausage Pizza Away)

The OSI (Open Systems Interconnection) model is a conceptual framework used to understand and design network systems.

Physical Layer:

This is the lowest layer, dealing with the physical connection between devices. It includes hardware elements like cables, switches, and network interface cards (NICs). Its main role is to transmit raw binary data (0s and 1s) over a physical medium.

Data Unit: Bits

Data Link Layer:

This layer is responsible for error detection and correction from the Physical Layer and for managing data frames. It ensures that data sent from the Network Layer is error-free and manages protocols for establishing, maintaining, and terminating connections.

The data link layer receives the information in the form of packets from the Network layer, it divides packets into frames and sends those frames bit-by-bit to the underlying physical layer.

It also attaches some special bits (for error control and addressing) at the header and end of the frame. At the receiver’s end, DLL takes bits from the Physical layer organizes them into the frame, and sends them to the Network layer.

Examples: Includes technologies like Ethernet, Wi-Fi, and ARP (Address Resolution Protocol), which maps IP addresses to MAC addresses.

Data Unit: Frames

Network Layer:

The Network Layer handles routing and forwarding of data packets between devices across different networks. It manages logical addressing (like IP addresses) and determines the best path for data to travel from source to destination.

Functions of this layer:

Packetizing

The process of encapsulating the data received from the upper layers of the network (also called payload) in a network layer packet at the source and decapsulating the payload from the network layer packet at the destination is known as packetizing.

Routing

Routing is the process of determining the path or route that data packets should take through the network from the source to the destination.

Protocols are OSPF (Open Shortest Path First), BGP (Border Gateway Protocol), and RIP (Routing Information Protocol).

Forwarding

Forwarding is the process of sending the packet from an incoming interface to an outgoing interface based on the routing table.

Congestion control

Congestion control is a network management process used to prevent network congestion, which occurs when the demand for network resources exceeds the available capacity, leading to packet loss, delays, and reduced overall network performance.

Examples: IP (Internet Protocol), ICMP (Internet Control Message Protocol)

Data Unit: Packets

Transport Layer (PORT):

This layer ensures reliable data transfer between end systems. It handles error recovery, flow control, and data segmentation.

Functions

At the sender’s side: The transport layer receives data (message) from the Application layer and then performs Segmentation, divides the actual message into segments, adds the source and destination’s port numbers into the header of the segment, and transfers the message to the Network layer.

At the receiver’s side: The transport layer receives data from the Network layer, reassembles the segmented data, reads its header, identifies the port number, and forwards the message to the appropriate port in the Application layer.

The Process to Process Delivery

End-to-End Connection between Hosts

Multiplexing and Demultiplexing

Congestion Control

Examples: TCP (Transmission Control Protocol), UDP (User Datagram Protocol)

Data Unit: Segments (TCP) / Datagrams (UDP)

Session Layer:

This layer manages sessions or connections between applications. It establishes, maintains, and terminates communication sessions, handling the setup and teardown of connections and managing dialogue control.

SSL/TLS interacts with the Session Layer as it manages and controls the sessions between two communicating endpoints. This includes establishing, maintaining, and terminating secure sessions.

Presentation Layer:

This layer translates data between the application layer and the network format. It handles data encryption, decryption, and translation services to ensure that data is in a readable format for the application layer.

SSL (Secure Sockets Layer) and its successor TLS (Transport Layer Security) are cryptographic protocols designed to secure communications over a network.

It ensures that the data transmitted between a client and server is encrypted so that it remains confidential and secure.

SSL/TLS Handshake Process

Client Hello:

The client initiates a connection with a "Client Hello" message that includes supported SSL/TLS versions, cipher suites, and a random value.

Server Hello:

The server responds with a "Server Hello" message, choosing the SSL/TLS version and cipher suite for the session. The server also provides its digital certificate for authentication.

Certificate Exchange:

The server sends its certificate to the client. The client verifies the server’s certificate against trusted Certificate Authorities (CAs).

Key Exchange:

The client generates a pre-master secret, encrypts it with the server’s public key, and sends it to the server. Both parties use this pre-master secret to derive a session key.

Finished Messages:

Both the client and server send encrypted "Finished" messages to confirm that the handshake was successful and that the session is secure.

Secure Communication:

Once the handshake is complete, the client and server use the established session key to encrypt and decrypt data during the communication session.

Application Layer:

The top layer, where end-user software and applications interact with the network. It provides network services directly to applications, such as email, file transfers, and web browsing.

Examples: HTTP (for web browsing), FTP (for file transfers), SMTP (for email), and DNS (for domain name resolution).

Wireless security protocols

It is used for protecting data transmitted over wireless networks from unauthorized access and attacks. It secures the data link layer (Layer 2) of the OSI model, focusing on securing the communication channel itself rather than the content of the communication.

WEP (Wired Equivalent Privacy)

It was one of the first security protocols designed for wireless networks. It was intended to provide a level of security comparable to wired networks. It uses RC4 stream cipher with 40-bit or 104-bit keys for encryption.

WPA (Wi-Fi Protected Access)

It uses TKIP (Temporal Key Integrity Protocol) for encryption.

WPA2 (Wi-Fi Protected Access 2)

It is an enhancement over WPA and has become the standard for wireless security.

It uses AES (Advanced Encryption Standard) for encryption

WPA3 (Wi-Fi Protected Access 3)

It is the latest standard, designed to improve security over WPA2.

Windows Registry

It is a hierarchical database used by the Windows operating system to store configuration settings and options for the operating system and installed applications.

How to Access the Registry?

Press Win + R to open the Run dialog.

Type regedit and press Enter.

Subnetting

It is a method used in IP networking to divide a larger network into smaller, more manageable sub-networks or subnets.

A subnet mask defines which part of the IP address is the network portion and which part is the host portion. It works by masking the IP address, helping to determine the network and host ranges.

Classless Inter-Domain Routing (CIDR) notation is used to specify IP addresses and their associated network prefix. For example, 192.168.1.0/24 means that the first 24 bits are the network portion.

Why Subnetting?

Imagine you are a network administrator tasked with setting up a network for a medium-sized company. The company has been allocated a block of IP addresses: 192.168.1.0/24. This means you have a total of 256 IP addresses available in this block (from 192.168.1.0 to 192.168.1.255).

Without subnetting, you might assign the entire 192.168.1.0/24 network to a single flat network. If you only have a few devices in this network, a large portion of the IP addresses will remain unused. All devices in the network will receive broadcast traffic (like ARP requests), which can lead to network congestion and reduced performance.

Solution with Subnetting

To address these issues, you can subnet the 192.168.1.0/24 network into smaller subnets based on the actual needs of different departments or segments of your network. Let’s assume you need four subnets: one for the Sales department, one for the HR department, one for the IT department, and one for guest Wi-Fi.

1) Determine the Number of Subnets Needed:

You need 4 subnets. To create 4 subnets, you need 2 additional bits (since 2^2 = 4).

2) Calculate the New Subnet Mask:

Original subnet mask for 192.168.1.0/24 is 255.255.255.0 (/24).

With 2 additional bits for subnetting, the new subnet mask is 255.255.255.192 (/26).

3) Calculate Subnet Details:

A /26 subnet provides 64 IP addresses (including network and broadcast addresses), with 62 usable IP addresses per subnet.

4) Create Subnets:

Subnet 1:

192.168.1.0/26

Network Address: 192.168.1.0

Broadcast Address: 192.168.1.63

Usable IP Range: 192.168.1.1 to 192.168.1.62

Subnet 2:

192.168.1.64/26

Network Address: 192.168.1.64

Broadcast Address: 192.168.1.127

Usable IP Range: 192.168.1.65 to 192.168.1.126

Subnet 3:

192.168.1.128/26

Network Address: 192.168.1.128

Broadcast Address: 192.168.1.191

Usable IP Range: 192.168.1.129 to 192.168.1.190

Subnet 4:

192.168.1.192/26

Network Address: 192.168.1.192

Broadcast Address: 192.168.1.255

Usable IP Range: 192.168.1.193 to 192.168.1.254

5) Assign Subnets to Departments:

Sales Department: 192.168.1.0/26

Use IPs 192.168.1.1 to 192.168.1.62

HR Department: 192.168.1.64/26

Use IPs 192.168.1.65 to 192.168.1.126

IT Department: 192.168.1.128/26

Use IPs 192.168.1.129 to 192.168.1.190

Guest Wi-Fi: 192.168.1.192/26

Use IPs 192.168.1.193 to 192.168.1.254

What can we infer from Subnet mask?

Ex. The subnet mask 255.255.252.0 in binary format is:

11111111.11111111.11111100.00000000

1. Determine the Prefix Length
   The prefix length, or the subnet mask length in CIDR notation, is the number of 1s in the binary subnet mask:
   So, the total number of 1s is 8 + 8 + 6 + 0 = 22.

Thus, the subnet mask 255.255.252.0 corresponds to a /22 prefix length.

1. Calculate the Number of Subnets and Hosts
   Network Portion: (22)
   Host Portion: (32 - 22 = 10)

Number of Subnets

If you are given a larger block and are subnetting it with a /22 mask, the number of subnets will depend on the original network size. For instance, if you start with a /16 network (e.g., 10.0.0.0/16) and subnet it into /22 subnets

A /16 network has 2^(22-16) = 2^6 = 64 subnets of /22.

Number of Hosts per Subnet

In each /22 subnet:

Total number of IP addresses = 2^10 = 1024.

Usable IP addresses = 1024 - 2 (network address and broadcast address) = 1022.

1. Subnet Address Range
   To understand the specific ranges of addresses in a /22 subnet, let’s consider an example:

Network Address: 192.168.0.0/22

The first subnet is from 192.168.0.0 to 192.168.3.255.

In this subnet:

Network Address: 192.168.0.0

Broadcast Address: 192.168.3.255

Usable IP Range: 192.168.0.1 to 192.168.3.254

Error Detection Methods

Simple Parity Check:

Two-Dimensional Parity Check

Checksum

Cyclic Redundancy Check (CRC)

Routing Protocol

Open Shortest Path First

Border Gateway Protocol (BGP)

Interior Gateway Routing Protocol (IGRP)

Enhanced Interior Gateway Routing Protocol (EIGRP)

Exterior Gateway Protocol (EGP):

4)Port:

It is a logical channel through which data can be sent/received to an application

It is a 16-bit integer, hence, we have 216 ports available which are categorized as shown below:

Well known Ports (0 – 1023)

Registered Ports (1024 – 49151)

Ephemeral Ports (49152 – 65535)

Number of ports: (65,536)

Range: (0 – 65535)

netstat -a

Communication modes

Unicasting

It is the process of sending a message from one source node to a single, specific destination node

Ex. P2P

Anycasting

It involves sending a message from a source to the nearest or best-suited node among a group of potential destination nodes

Ex. Content Delivery Networks (CDNs) and DNS services

Multicasting

It involves sending a message from a source to multiple specified nodes (a group) simultaneously

Broadcasting

It sends a message from a source to all nodes in a network segment

Ex. ARP (Address Resolution Protocol) and DHCP (Dynamic Host Configuration Protocol)

Repeaters: Networking devices used to amplify or regenerate signals in a network. They are used to extend the range of communication over long distances by boosting the signal strength and compensating for signal degradation.

Transmission Media

It refers to the physical path through which data is transmitted from one device to another in a network

Guided (Wired) Transmission Media:

Twisted Pair Cables:

They consist of pairs of copper wires twisted together to reduce electromagnetic interference (EMI) and crosstalk (interference between adjacent wires).

Coaxial cables

They consist of a single central conductor surrounded by layers of insulation and shielding, which provides protection against external interference.

Universal Serial Bus (USB)

These are widely used for connecting computers and peripheral devices, such as printers, keyboards, mice, and external storage devices. They can also be used for charging devices like smartphones and tablets.

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