Computer Networks Notes — BCA Sem 4

Computer Networks — Introduction

Network: Do ya zyada computers jo data share karne ke liye connected hain.

Benefits:

• Resource sharing (printer, files, internet)
• Communication (email, video call)
• Data backup & centralized storage
• Cost reduction

Network Types: | Type | Coverage | Example | |------|----------|---------| | PAN | Room | Bluetooth | | LAN | Building/campus | Office Wi-Fi | | MAN | City | Cable TV network | | WAN | Country/World | Internet |

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1. OSI Model — 7 Layers

Layer 7: Application   → HTTP, FTP, SMTP, DNS, Telnet
Layer 6: Presentation  → SSL/TLS, JPEG, ASCII, Encryption
Layer 5: Session       → Session management, NetBIOS
Layer 4: Transport     → TCP, UDP (Port numbers)
Layer 3: Network       → IP, ICMP, Routing
Layer 2: Data Link     → Ethernet, MAC addresses, ARP, Switch
Layer 1: Physical      → Cables, Wi-Fi, bits as electrical/optical signals

Mnemonic (top to bottom): "All People Seem To Need Data Processing"

Data Encapsulation:

Application → Data
Transport   → Segment (+ port numbers)
Network     → Packet (+ IP addresses)
Data Link   → Frame (+ MAC addresses)
Physical    → Bits (0s and 1s)

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2. TCP/IP Model (Practical)

Application Layer  → Application + Presentation + Session (OSI)
Transport Layer    → Transport (TCP/UDP)
Internet Layer     → Network (IP)
Network Access     → Data Link + Physical

TCP (Transmission Control Protocol)

3-Way Handshake (Connection Setup):
Client ──── SYN ──────────► Server
Client ◄─── SYN-ACK ──────── Server
Client ──── ACK ──────────► Server
[Connected! Data transfer begins]

4-Way Termination:
Client ──── FIN ──────────► Server
Client ◄─── ACK ──────────── Server
Client ◄─── FIN ──────────── Server
Client ──── ACK ──────────► Server

TCP features: Sequence numbers, acknowledgments, flow control (sliding window), congestion control, error detection.

UDP (User Datagram Protocol)

Header: Source Port | Dest Port | Length | Checksum
(Only 8 bytes!)

No connection, no guarantee, just speed.
Use: DNS, DHCP, Video streaming, Online gaming

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3. IP Addressing

IPv4 Classes

Class A: 1.0.0.0   – 126.255.255.255  /8  (large orgs)
Class B: 128.0.0.0 – 191.255.255.255  /16 (medium orgs)
Class C: 192.0.0.0 – 223.255.255.255  /24 (small networks)
Class D: 224.0.0.0 – 239.255.255.255  Multicast
Class E: 240.0.0.0 – 255.255.255.255  Reserved

Private Ranges (not on internet):
10.0.0.0/8
172.16.0.0 – 172.31.255.255 (/12)
192.168.0.0/16

Subnetting Example

Network: 192.168.1.0/24
Subnet Mask: 255.255.255.0
Hosts: 2^8 – 2 = 254

Split into 4 subnets → borrow 2 bits → /26
New mask: 255.255.255.192

Subnet 1: 192.168.1.0/26   → hosts .1–.62,  broadcast .63
Subnet 2: 192.168.1.64/26  → hosts .65–.126, broadcast .127
Subnet 3: 192.168.1.128/26 → hosts .129–.190, broadcast .191
Subnet 4: 192.168.1.192/26 → hosts .193–.254, broadcast .255

Hosts per subnet: 2^6 – 2 = 62

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4. Application Layer Protocols

HTTP/HTTPS

Request:
GET /notes HTTP/1.1
Host: wohotech.in
User-Agent: Chrome/120

Response:
HTTP/1.1 200 OK
Content-Type: text/html
[page content]

Common Status Codes:
200 OK, 201 Created, 301 Redirect
400 Bad Request, 401 Unauthorized, 403 Forbidden
404 Not Found, 500 Internal Server Error

DNS Resolution Steps

1. Browser checks own cache
2. OS checks /etc/hosts file
3. Query → Local DNS Resolver (ISP)
4. Resolver checks cache → if miss:
5. Query Root DNS server (.)
6. Root → .com TLD server
7. TLD → google.com Authoritative server
8. Authoritative → returns 142.250.x.x
9. Resolver caches → returns to browser
10. Browser connects to IP

Email Protocols

SMTP (port 25/587): Send mail
POP3 (port 110): Download mail, delete from server
IMAP (port 143): Access mail, stays on server, sync across devices

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5. Network Devices

| Device | OSI Layer | Function | |--------|-----------|----------| | Hub | Physical (L1) | Broadcast to all ports | | Switch | Data Link (L2) | Forward by MAC address | | Router | Network (L3) | Route by IP address | | Firewall | L3/L4/L7 | Filter traffic by rules | | Access Point | L1/L2 | Wireless connectivity | | Modem | L1 | Convert digital↔analog |

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6. Routing Protocols

Static Routing: Admin manually configures routes
  ip route 10.0.0.0 255.0.0.0 192.168.1.1

Dynamic Routing:
  RIP (Routing Information Protocol):
    Distance vector, metric = hop count, max 15 hops
    Updates every 30 seconds, slow convergence

  OSPF (Open Shortest Path First):
    Link state, Dijkstra algorithm, metric = cost
    Fast convergence, scalable, used in enterprise

  BGP (Border Gateway Protocol):
    Path vector, used between ISPs (Internet backbone)
    Policy-based, autonomous system routing

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7. Network Security Basics

Common Attacks

Phishing:     Fake emails/websites steal credentials
MITM:         Intercept communication between two parties
DDoS:         Flood server with traffic → crash
SQL Injection: Malicious SQL in input fields
Packet Sniffing: Capture unencrypted network traffic
ARP Spoofing: Fake ARP replies → redirect traffic

Defenses

Firewall:    Block/allow traffic by rules (port, IP, protocol)
IDS/IPS:     Detect/prevent intrusions
Encryption:  HTTPS (TLS), VPN, SSH
Authentication: Strong passwords, MFA, certificates
Updates:     Patch known vulnerabilities regularly

Firewall Rules (iptables example)

# Allow established connections
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allow HTTP/HTTPS
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j ACCEPT

# Allow SSH from specific IP only
iptables -A INPUT -p tcp -s 192.168.1.100 --dport 22 -j ACCEPT

# Drop all other incoming
iptables -A INPUT -j DROP

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Exam Important Questions

1. OSI model ke 7 layers explain karo — har layer ka kaam
2. TCP ka 3-way handshake diagram ke saath explain karo
3. IP subnetting: 192.168.10.0/24 ko 8 equal subnets mein divide karo
4. DNS resolution process step-by-step
5. TCP aur UDP comparison table
6. Router aur Switch mein kya difference hai?
7. RIP aur OSPF mein kya difference hai?
8. Common network attacks aur unse bachne ke tarike

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Viva Questions

Q: MAC address aur IP address mein kya fark hai? A: MAC (Media Access Control): hardware address, factory mein assigned, 48-bit hex (AA:BB:CC:DD:EE:FF), same LAN ke andar communication. IP: logical address, software assigned, changes with network, routing ke liye use hota hai.

Q: DHCP kya karta hai? A: Dynamic Host Configuration Protocol — automatically IP address, subnet mask, gateway, DNS server assign karta hai. Koi manually configure nahi karna padta. Server → broadcast → client request → server offers IP → client accepts → server confirms (DORA process).

Q: Ping aur Traceroute mein kya difference hai? A: Ping: ICMP echo request/reply, connectivity test, RTT measure. Traceroute: TTL badha ke har hop ka IP pata karta hai — poora path map ho jaata hai source se destination tak.