1. Introduction to Computer Networks

A computer network is a collection of interconnected computers and devices that communicate with each other and share resources.

Main objectives of networking

• Resource sharing
• Data sharing
• Communication
• Centralized management
• Reliability
• Scalability
• Cost reduction

Important network components

1. Sender – Device that sends data.
2. Receiver – Device that receives data.
3. Transmission Medium – Path through which data travels.
4. Network Interface Card (NIC) – Connects a device to a network.
5. Switch – Connects devices within a LAN.
6. Router – Connects different networks.
7. Gateway – Connects networks using different protocols.
8. Modem – Converts digital and analog signals.

2. OSI Reference Model

The OSI (Open Systems Interconnection) model was developed by the International Organization for Standardization (ISO).

It consists of 7 layers.

OSI Layers

From top to bottom:

1. Application
2. Presentation
3. Session
4. Transport
5. Network
6. Data Link
7. Physical

Mnemonic

All People Seem To Need Data Processing

• A – Application
• P – Presentation
• S – Session
• T – Transport
• N – Network
• D – Data Link
• P – Physical

Layer 7 – Application Layer

Provides network services directly to applications.

Examples

• HTTP/HTTPS
• FTP
• SMTP
• DNS
• Telnet

It is the layer closest to the end user.

Layer 6 – Presentation Layer

Responsible for data representation.

Functions:

• Translation
• Encryption/decryption
• Compression/decompression

Examples:

• ASCII
• JPEG
• MPEG
• Encryption formats

Layer 5 – Session Layer

Responsible for establishing, managing and terminating sessions between applications.

Functions:

• Session establishment
• Session management
• Synchronization
• Session termination

Layer 4 – Transport Layer

Provides end-to-end communication.

Major protocols:

• TCP
• UDP

Functions:

• Segmentation
• Flow control
• Error control
• Reliability
• Port addressing

TCP

TCP is:

• Connection-oriented
• Reliable
• Ordered
• Error-controlled

UDP

UDP is:

• Connectionless
• Faster
• Unreliable
• No guarantee of delivery

Layer 3 – Network Layer

Responsible for logical addressing and routing.

Main device:

Router

Important protocol:

IP

Functions:

• Routing
• Logical addressing
• Packet forwarding
• Path determination

Layer 2 – Data Link Layer

Responsible for reliable node-to-node delivery.

Functions:

• Framing
• MAC addressing
• Error detection
• Flow control

Devices:

• Switch
• Bridge

The Data Link Layer has two sublayers:

LLC

Logical Link Control

MAC

Media Access Control

Layer 1 – Physical Layer

Deals with transmission of raw bits.

Functions:

• Bit transmission
• Cable specifications
• Voltage
• Connectors
• Data rates
• Physical topology

Examples:

• Ethernet cables
• Fiber optic cable
• Radio signals

3. OSI Layer – PDU

Important: TCP uses Segment, IP uses Packet/Datagram, and Data Link uses Frame.

4. TCP/IP Model

The TCP/IP model is the practical networking model used on the Internet.

The commonly taught TCP/IP model has 4 layers:

1. Application
2. Transport
3. Internet
4. Network Access

OSI vs TCP/IP

Important TCP/IP protocols

Application Layer

• HTTP
• HTTPS
• FTP
• SMTP
• DNS
• DHCP
• SSH

Transport Layer

• TCP
• UDP

Internet Layer

• IP
• ICMP
• ARP is commonly associated with network access/link functions, though its placement varies by model.

Network Access

• Ethernet
• Wi-Fi

5. OSI vs TCP/IP

6. LAN, MAN and WAN

LAN – Local Area Network

A network covering a small geographical area.

Examples:

• Home
• School
• Office
• Computer laboratory

Characteristics:

• High speed
• Limited geographical area
• Usually privately managed

Example:

Ethernet

MAN – Metropolitan Area Network

Covers a city or metropolitan area.

Example:

A network connecting multiple branches of an organization across a city.

WAN – Wide Area Network

Covers a large geographical area.

Examples:

• Internet
• Inter-city networks
• International networks

WAN generally has greater latency than LAN.

7. IP Addressing

An IP address identifies a device/interface logically on an IP network.

There are two major versions:

• IPv4
• IPv6

IPv4

IPv4 uses 32-bit addresses.

Example:

192.168.1.10

IPv4 consists of four octets.

Each octet contains 8 bits.

Therefore:

4 × 8 = 32 bits

IPv4 range

0.0.0.0 to 255.255.255.255

Total addresses:

2³² = 4,294,967,296

8. IPv4 Address Classes

Traditional classful addressing divides IPv4 into five classes.

Important

127.x.x.x is reserved for loopback.

Example:

127.0.0.1

9. Private IPv4 Addresses

Private addresses are commonly used inside local networks.

Class A private range

10.0.0.0 – 10.255.255.255

Class B private range

172.16.0.0 – 172.31.255.255

Class C private range

192.168.0.0 – 192.168.255.255

These addresses are not directly routable on the public Internet.

10. IPv6

IPv6 uses 128-bit addresses.

Example:

2001:db8::1

Advantages:

• Huge address space
• Better support for modern networking
• Simplified header
• Improved multicast support
• No traditional IPv4 broadcast

Total possible IPv6 addresses:

2¹²⁸

11. MAC Address

MAC stands for Media Access Control.

A MAC address is a hardware/link-layer address associated with a network interface.

A typical MAC address is 48 bits.

Example:

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

IP vs MAC

12. Routing

Routing is the process of determining a path for packets from source to destination.

Routers maintain routing information in a routing table.

Routing types

1. Static Routing
2. Dynamic Routing

Static Routing

Routes are manually configured by a network administrator.

Advantages:

• Simple for small networks
• Predictable
• No routing protocol overhead

Disadvantages:

• Difficult to maintain in large networks
• Does not automatically adapt to topology changes

13. Dynamic Routing

Routes are learned and updated automatically.

Major categories:

Distance Vector

Routers determine paths using distance and direction information.

Example:

RIP

Link State

Routers maintain information about network topology.

Examples:

• OSPF
• IS-IS

Path Vector

Used for inter-domain routing.

Example:

BGP

14. Routing Algorithms

14.1 Distance Vector Algorithm

Each router maintains information about:

• Destination
• Distance/cost
• Next hop

Routers exchange routing information with neighboring routers.

A famous example is:

RIP

RIP uses hop count as its routing metric.

Maximum usable hop count:

15

A hop count of 16 represents unreachable in RIP.

15. Link State Routing

In link-state routing:

1. Routers discover neighbors.
2. Determine link costs.
3. Distribute link-state information.
4. Build a topology database.
5. Calculate shortest paths.

A commonly used algorithm is:

Dijkstra's Shortest Path First algorithm

OSPF uses the SPF approach.

16. BGP

BGP = Border Gateway Protocol

It is the major routing protocol used for routing between autonomous systems on the Internet.

BGP is a path-vector protocol.

It uses TCP for communication.

BGP uses:

TCP port 179

17. LAN Technologies

Ethernet

Ethernet is one of the most widely used LAN technologies.

IEEE standard:

802.3

Wi-Fi

Wi-Fi is based on the IEEE 802.11 family of standards.

18. Network Topologies

Bus

All devices share a common communication medium.

Star

All devices connect to a central device.

Common in Ethernet LANs.

Ring

Devices form a circular structure.

Mesh

Devices have multiple interconnections.

Tree

Hierarchical combination of network segments.

19. Network Security

Network security protects:

• Data
• Devices
• Networks
• Services
• Users

from unauthorized access, attacks and misuse.

20. CIA Triad

The three fundamental principles of information security are:

Confidentiality

Only authorized people should access information.

Integrity

Information should not be improperly modified.

Availability

Authorized users should be able to access resources when required.

CIA = Confidentiality + Integrity + Availability

21. Firewall

A firewall monitors and controls network traffic according to security rules.

It can control:

• Incoming traffic
• Outgoing traffic
• Ports
• Protocols
• IP addresses

22. Encryption

Encryption converts plaintext into ciphertext.

Symmetric Encryption

Uses the same key for encryption and decryption.

Examples:

• AES
• DES

Asymmetric Encryption

Uses a pair of keys:

• Public key
• Private key

Example:

RSA

23. Hashing

Hashing converts data into a fixed-length digest.

Examples:

• SHA-256
• SHA-3

Hash functions are commonly used for:

• Integrity verification
• Password protection systems
• Digital signatures

24. Digital Signature

A digital signature helps provide:

• Authentication
• Integrity
• Non-repudiation

It uses asymmetric cryptographic techniques.

25. Common Network Attacks

DoS

Denial of Service

Attempts to make a service unavailable.

DDoS

Distributed Denial of Service

A DoS attack launched from multiple compromised systems.

Phishing

Attempts to trick users into revealing sensitive information.

Spoofing

Attacker disguises identity or source information.

Man-in-the-Middle

Attacker intercepts communication between two parties.

Malware

Malicious software such as:

• Virus
• Worm
• Trojan
• Ransomware

26. Important Network Protocols

50+ Important MCQs

MCQ 1

How many layers are present in the OSI model?

A. 4
B. 5
C. 7
D. 8

Answer: C. 7

MCQ 2

Which layer of the OSI model is responsible for routing?

A. Physical
B. Data Link
C. Network
D. Transport

Answer: C. Network

MCQ 3

Which OSI layer is responsible for logical addressing?

A. Physical
B. Network
C. Session
D. Presentation

Answer: B. Network

MCQ 4

Which device primarily operates at the Network Layer?

A. Hub
B. Switch
C. Router
D. Repeater

Answer: C. Router

MCQ 5

Which layer is responsible for framing?

A. Network
B. Transport
C. Data Link
D. Physical

Answer: C. Data Link

MCQ 6

Which OSI layer transmits raw bits?

A. Physical
B. Data Link
C. Network
D. Transport

Answer: A. Physical

MCQ 7

Which layer provides end-to-end delivery?

A. Network
B. Transport
C. Session
D. Data Link

Answer: B. Transport

MCQ 8

TCP is a:

A. Connectionless protocol
B. Connection-oriented protocol
C. Physical protocol
D. Routing algorithm

Answer: B. Connection-oriented protocol

MCQ 9

UDP is:

A. Connection-oriented
B. Connectionless
C. Circuit-switched
D. Hardware-based

Answer: B. Connectionless

MCQ 10

Which OSI layer is responsible for encryption and compression?

A. Application
B. Presentation
C. Session
D. Transport

Answer: B. Presentation

MCQ 11

Which layer establishes and manages sessions?

A. Session
B. Transport
C. Network
D. Application

Answer: A. Session

MCQ 12

How many layers are commonly defined in the TCP/IP model?

A. 3
B. 4
C. 5
D. 7

Answer: B. 4

MCQ 13

Which protocol is used for domain-name resolution?

A. FTP
B. DNS
C. SMTP
D. DHCP

Answer: B. DNS

MCQ 14

Which protocol automatically assigns IP configuration to hosts?

A. DNS
B. HTTP
C. DHCP
D. FTP

Answer: C. DHCP

MCQ 15

Which protocol is used to send email?

A. SMTP
B. FTP
C. DNS
D. ARP

Answer: A. SMTP

MCQ 16

IPv4 addresses are:

A. 16 bits
B. 32 bits
C. 64 bits
D. 128 bits

Answer: B. 32 bits

MCQ 17

IPv6 addresses are:

A. 32 bits
B. 64 bits
C. 128 bits
D. 256 bits

Answer: C. 128 bits

MCQ 18

Which is a valid private IPv4 address?

A. 8.8.8.8
B. 192.168.1.10
C. 1.1.1.1
D. 172.40.1.1

Answer: B. 192.168.1.10

MCQ 19

What is the loopback IPv4 address commonly used?

A. 127.0.0.1
B. 192.168.1.1
C. 10.0.0.1
D. 255.255.255.255

Answer: A. 127.0.0.1

MCQ 20

Which IPv4 class is used for multicast?

A. Class A
B. Class B
C. Class C
D. Class D

Answer: D. Class D

MCQ 21

Class C IPv4 addresses traditionally have which default subnet mask?

A. 255.0.0.0
B. 255.255.0.0
C. 255.255.255.0
D. 255.255.255.255

Answer: C. 255.255.255.0

MCQ 22

Which address is in the private Class A range?

A. 10.5.6.7
B. 11.5.6.7
C. 127.5.6.7
D. 172.40.1.1

Answer: A. 10.5.6.7

MCQ 23

A MAC address is generally:

A. 16 bits
B. 32 bits
C. 48 bits
D. 128 bits

Answer: C. 48 bits

MCQ 24

Which device forwards packets between different networks?

A. Hub
B. Router
C. Repeater
D. NIC

Answer: B. Router

MCQ 25

Which technology is associated with IEEE 802.3?

A. Bluetooth
B. Ethernet
C. Wi-Fi
D. Token Ring

Answer: B. Ethernet

MCQ 26

IEEE 802.11 is primarily associated with:

A. Ethernet
B. Wi-Fi
C. Bluetooth<br dat