Objective of computer networks:
The objective of computer networks is to enable resource sharing, data communication and collaboration.

• Resource sharing:
  Computer networks allow devices to share resources such as printers and storage, reducing costs and improving efficiency.

Example:-
In an office network, multiple computer can share a single printer, reducing the need for multiple printers.

• Data communication:
  Network facilitate data transfer, enabling communication through emails, instant messaging, and video conferencing.

Example:-
Employees in different locations can collaborate through video conferencing tools like Zoom or Microsoft Teams.

To compute the transmission time, we use formula

Time = Message Size (in bits)
       Data Rate (in bps)

Given:
Data rate = 500 bps
1 kilobit (kb) = 1000 bits
1 kilobytes (KB) = 8 x 1000 = 8000 bits

a) it is of 10 kilobitsTime

= (10,000 bits)/500 bps     

= 20 seconds

b) it is of 10 kilobytesTime 

= (10,000 x 8 bits)/500 bps

= 160 seconds.
       

Transmission Methods:
Data can be transmitted across a computer network using packet switching or circuit switching. 

Packet Switching:
Packet switching divides data into a small packets before sending them across a network. Each packet travels independently and may take different routes to reach the destination, where they are reassembled into the original message.

Process:
_-The data is broken into packets (small chunks)
_-Each packet contains a header (source, destination, sequence number, etc), and payload (actual data).
_-Packets are sent independently across the network.
_-Packets may take different paths based on network conditions.
-The destination device reorders packets and reconstructs the original message.

Circuit Switching:
Circuit switching establishes a dedicated communication path between the sender and receiver before data transmission begins. This path remains open for the entire session.

Process:
_-A connection (circuit) is established between sender and receiver.
_-Data flows continuously along the path.
_-After transmission, the connection is terminated.

Packet Switching is ideal for modern internet-based communication while Circuit Switching is still used in traditional voice calls but is less common for data transfers.

Protocol:-
Protocols are sets of rules that govern data communication. A protocol is a formal agreement arrangement between two computers send and receive information.

i) TCP/IP (Transmission Control Protocol/Internet Protocol):
Each layer of TCP/IP model has its own protocol(s). Every protocol is designed to perform some specific task.

ii) HTTP:
Hypertext Transfer Protocol is a protocol used by World Wide Web (WWW) to transfer webpages between a client and a webserver. A webserver is also called called an HTTP server.

iii) DNS (Domain Name System):-
DNS translates domain name to IP addresses, making it easier for users to access websites.

Example:
When you type www.example.com in a web browser DNS translates it to the corresponding IP address.

iv) DHCP(Dynamic Host Configuration Protocol):-
DHCP automatically assigns IP addresses to devices on a network, simplifying network management.

Introduction to Network Security:
Network security protects data from unauthorized access, cyber threats, and attacks. Below are three key security methods.

• Firewalls:-
  A firewall acts as a security barrier between a private network and external threats. It monitors and controls incoming and outgoing traffic based on predefined security rules. Firewalls prevent unauthorized access but cannot detect threats hidden in encrypted traffic.
• Encryption:- It protects data by converting it into a secure format, ensuring that only authorized users can decode it. It is essential for secure online communication, such as banking and messaging. However, encryption requires effective key management as losing the decryption key makes the data inaccessible.
• Antivirus Software:-
  Antivirus software detects and removes malicious software. It provides real-time protection, but it needs frequent updates to stay effective against new threats. Despite its benefits, it cannot prevent attacks like phishing or social engineering.

Introduction to Applications:
Computer network helps people to connect, share information and work efficiently. They are widely used in business, education, and healthcare to improve communication and services.

• Business:-
  In Business, networks enables efficient communication, resource sharing, and data management.

Example:-
Companies use internet to share information and resources securely within the organization.

• Education:-
  Educational institutions use networks to provide online learning platforms, virtual classrooms, and access to educational resources.

Example:-
Universities use Learning Management System (LMS) like Blackboard and Moodle to deliver course content and assessments.

• Healthcare:-
  Healthcare networks facilitate the sharing of patient information, telemedicine, and access to medical database.

Example:-
Hospitals uses electronic health records (EHR) system to store and retrieve patient data effecti efficiently.

Star Topology:

• Structure: All devices connect to a central hub.
• Failure Impact: If the hub fails, the entire network goes down.
• Scalability: Easy to add/remove device without effecting the network.
• Cost: Moderate, requires a hub.
• Best use Cases: Office networks, LANs.

Ring Topology:

• Structure: Devices are connected in a closed loop.
• Failure Impact: If one device fails, the entire network can be disrupted.
• Scalability: Difficult to expand without breaking the ring.
• Performance: Moderate, as data passes through multiple devices before reaching the destination.
• Cost: Low, but expensive for large networks.
• Best use cases: Older Token Ring networks.

Bus Topology:

• Structure: A single backbone cable connects all devices.
• Failure Impact: If the main cable fails, the whole network stops working.
• Scalability: limited by cable length and signal degradation.
• Performance: Low, as all devices share bandwidth.
• Cost: Low, as minimal cabling is needed.
• Best use cases: Small networks with few devices.

Mesh topology:

• Structure: Each device is connected to everyother device.
• Failure Impact: Highly reliable, failure of one link doesnot effect the network.
• Scalability: Difficult to expand due to high cost and complexity.
• Performance: Very high, as multiple paths ensures smooth data flow.
• Cost: Very high, due to extensive cabling and configuration.
• Best use cases: High security environment like banking and military networks.

Definition of Shift Cipher:
A shift cipher (Caesar cipher) shifts each letter in the alphabet by a fixed amount. In this case, the shift amount is 4.

a) Encrypt "SECURITY"

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
E F G H I J K L M N O P Q R S T U V W X Y Z A B C D

The word 'SECURITY' Encrypted in "WIGYVMX"

b) Decrypt "WMXYVMI"

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
E F G H I J K L M N O P Q R S T U V W X Y Z A B C D

The word "WMXYVMI" decrypt in "SITURIE"

Part a): Total IPv4 Addresses: 

An IPv4 address is a 32-bit number meaning there are 2³² possible addresses
2³² =  4,294,967,296
So, the total number of unique IPv4 addresses is 4,294,967,296. 

Part b): Reserved Addresses Calculation:

If 10% of the addresses are reserved for special purposes:

10%  of 4,294,967,296 = 10​/100 x 4,294,967,296 = 429, 496,729.6

Since number of addresses must be whole, we around to 429,496,729.

Now the remaining usable addresses:
4,294,967,296 - 429,496,729 = 3,865,470,567

So, 3,865,470,567 IPv4 addresses are left for general use.