Computer Networks
Complete Notes

What is a Network?

A computer network is a collection of interconnected computers that communicate and share resources through communication protocols.

Types of Networks

Network Models

• Peer-to-Peer (P2P): All computers equal, no central server
• Client-Server: Central server serves multiple clients
• Hybrid: Combination of P2P and client-server

Network Topologies

OSI Model Overview

The Open Systems Interconnection (OSI) model is a 7-layer framework that standardizes network communication.

The 7 Layers

Data Encapsulation

Application Layer (Data)
        ↓ Adds header
Transport Layer (Segment)
        ↓ Adds header
Network Layer (Packet)
        ↓ Adds header
Data Link Layer (Frame)
        ↓ Adds header
Physical Layer (Bits)

On receiving: Remove headers layer by layer (decapsulation)

Data Link Layer Functions

• Framing: Group bits into frames
• MAC Addressing: Identify devices on LAN
• Error Detection: CRC (Cyclic Redundancy Check)
• Media Access Control: Decide who sends next

MAC Address

48-bit address identifying devices on local network. Format: 6 pairs of hexadecimal (e.g., 00:1A:2B:3C:4D:5E)

First 3 octets (24 bits): Manufacturer ID (OUI)
Last 3 octets (24 bits): Device-specific ID

Example: 08:00:27:2C:2F:65
08:00:27 = Oracle VirtualBox
2C:2F:65 = Individual device

Ethernet Protocol

Media Access Control (MAC)

• CSMA/CD: Listen before sending, back off if collision
• Token Ring: Pass token to decide who sends
• Wireless (CSMA/CA): Collision avoidance

Network Layer Functions

• Routing: Find best path from source to destination
• IP Addressing: Logical addresses for networks
• Forwarding: Move packets along the route
• Congestion Control: Manage network traffic

IPv4 Addressing

IPv4 Address: 192.168.1.1
Each octet: 0-255 (8 bits)
Total: 32 bits = 2^32 = 4.3 billion addresses

Classes:
Class A: 1-126 (16.7 million hosts per network)
Class B: 128-191 (65,000 hosts)
Class C: 192-223 (256 hosts)
Class D: 224-239 (Multicast)
Class E: 240-255 (Reserved)

Subnetting

Divide large network into smaller subnets using subnet mask.

IP: 192.168.1.100 / 24
Subnet Mask: 255.255.255.0
Network Address: 192.168.1.0
Broadcast: 192.168.1.255
Usable IPs: 192.168.1.1 - 192.168.1.254

IPv6

128-bit address (2^128 addresses). Format: xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx

Example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
Shorthand: 2001:db8:85a3::8a2e:370:7334

ICMP & Ping

Internet Control Message Protocol for diagnostics. Ping tests reachability.

Transport Layer Functions

• End-to-end communication
• Port numbers (0-65535) identify applications
• Reliability (TCP) or Speed (UDP)
• Flow control and error handling

TCP vs UDP

TCP 3-Way Handshake

Client                      Server
  |------ SYN (seq=x) ----->|
  |<---- SYN-ACK (seq=y) ----|
  |---- ACK (seq=x+1) ----->|
  |
  [Connection Established]

Well-Known Ports

• 20/21: FTP
• 22: SSH
• 23: Telnet
• 25: SMTP (Email)
• 53: DNS
• 80: HTTP
• 443: HTTPS
• 3306: MySQL

Common Application Protocols

DNS (Domain Name System)

Converts domain names to IP addresses. Hierarchical distributed system.

Query Flow:
User → Local DNS Resolver → Root Nameserver
        → TLD Nameserver (.com, .org)
        → Authoritative Nameserver
        → IP Address returned

HTTP Methods

• GET: Retrieve data
• POST: Submit data
• PUT: Update resource
• DELETE: Remove resource
• HEAD: Like GET but no body

Email Protocols

• SMTP: Sending mail (port 25)
• POP3: Receiving (deletes after download)
• IMAP: Receiving (keeps on server)

Routing Types

• Static: Manually configured routes
• Dynamic: Automatically adjusts to network changes

Routing Algorithms

Routing Table

Destination | Netmask       | Gateway    | Interface | Metric
10.0.0.0    | 255.255.255.0 | 0.0.0.0    | eth0      | 0
10.0.1.0    | 255.255.255.0 | 10.0.0.1   | eth0      | 1
192.168.0.0 | 255.255.0.0   | 10.0.0.1   | eth0      | 2

Switching Basics

Switches forward frames based on MAC addresses. Build MAC address table dynamically.

MAC Address Table

MAC Address        | Port | TTL
00:11:22:33:44:55 | 1    | 300
AA:BB:CC:DD:EE:FF | 3    | 300
11:22:33:44:55:66 | 2    | 300

Switch learns addresses by observing source MACs
Ages out entries after timeout (usually 5 min)

Switching Methods

VLAN (Virtual LAN)

Logically segment network into separate VLANs. Same physical switch, different networks.

Spanning Tree Protocol (STP)

Prevents loops in switched networks. Blocks redundant paths.

Security Goals

• Confidentiality: Keep data private
• Integrity: Ensure data not modified
• Authentication: Verify identity
• Non-repudiation: Can't deny sending

Common Attacks

Encryption Methods

• Symmetric: Same key to encrypt/decrypt (AES, DES)
• Asymmetric: Public key to encrypt, private to decrypt (RSA)
• Hash: One-way function (MD5, SHA-256)

Firewalls & VPN

• Firewall: Filters traffic based on rules
• VPN: Encrypted tunnel for secure communication
• IDS/IPS: Intrusion detection/prevention

Wireless Standards

WiFi Security

• WEP: Deprecated, easily cracked
• WPA: Improved encryption
• WPA2/WPA3: Current standard, AES encryption

Wireless Channels

2.4 GHz: 13 channels (1-13), overlapping. Non-overlapping: 1, 6, 11

5 GHz: Many non-overlapping channels for less interference

Mobile Networks

• 3G: Mobile broadband (WCDMA, CDMA2000)
• 4G/LTE: High speed (100+ Mbps)
• 5G: Ultra high speed, low latency