Unit 2: Computer Networking Basics

1. Foundations of Computer Networks

1.1 Definition and Goals

A Computer Network is a set of computers connected together for the purpose of sharing resources. The connection between computers can be done via cabling, usually the Ethernet cable, or wirelessly through radio waves.

• Key Goals:
  • Resource Sharing: Sharing printers, storage devices, and files.
  • High Reliability: Replication of files on multiple machines ensures data availability even if one fails.
  • Scalability: Ability to increase system performance gradually by adding more processors.
  • Communication Medium: Facilitating email, chat, and video conferencing.

1.2 Classification by Geographical Scope

1. PAN (Personal Area Network): Smallest scope (e.g., Bluetooth connecting a phone to headphones). Range: ~10 meters.
2. LAN (Local Area Network): Covers a small geographic area like a home, office, or building. High data transfer rates. Ethernet/Wi-Fi are standard.
3. MAN (Metropolitan Area Network): Connects LANs across a city (e.g., Cable TV network).
4. WAN (Wide Area Network): Spans large physical distances (countries/continents). The Internet is the largest WAN.
5. SAN (Storage Area Network): A specialized, high-speed network that provides block-level network access to storage.

1.3 Data Switching Techniques

How data moves from source to destination.

• Circuit Switching: A dedicated physical path is established before data transfer (e.g., old telephone networks).
  • Pros: Constant bandwidth, no delay once established.
  • Cons: Inefficient use of resources during silence/idling.
• Packet Switching: Data is broken into small chunks called "packets." Each packet travels independently, potentially taking different routes.
  • Pros: Efficient line usage, robust against line failure.
  • Cons: Variable delay (jitter), requires reassembly at destination.

2. Network Topologies and Transmission Medium

2.1 Network Topologies

The geometric arrangement of devices (nodes) and links in a network.

2.2 Transmission Media

The physical path between transmitter and receiver.

A. Guided Media (Wired)

1. Twisted Pair Cable:
   • UTP (Unshielded Twisted Pair): Common in LANs (Cat5e, Cat6). Cheap, flexible, prone to EMI (Electromagnetic Interference).
   • STP (Shielded Twisted Pair): Has metal shielding. Better protection against interference, more expensive.
2. Coaxial Cable: Central copper conductor surrounded by insulation and a metal shield. Used in Cable TV and early Ethernet. High bandwidth, resistant to noise.
3. Fiber Optic Cable: Transmits data as light pulses using glass/plastic strands.
   • Single-Mode: Long distance, laser light source, small core.
   • Multi-Mode: Shorter distance, LED source, larger core.
   • Note: Immune to electromagnetic interference, highest speed, expensive.

B. Unguided Media (Wireless)

1. Radio Waves: Omnidirectional; can penetrate walls. Used for AM/FM radio.
2. Microwaves: Unidirectional (line-of-sight required). Used for cellular phones, satellite communications, and Wi-Fi.
3. Infrared: Short-range communication (TV remotes). Cannot penetrate walls.

3. Understanding OSI and TCP/IP Models

3.1 The OSI Model (Open Systems Interconnection)

A conceptual 7-layer model developed by ISO to standardize network communication.

Mnemonic: Please Do Not Throw Sausage Pizza Away (Physical to Application).

3.2 The TCP/IP Model

A practical model on which the Internet is built. It condenses the OSI layers.

1. Application Layer: Combines OSI Application, Presentation, and Session layers.
2. Transport Layer: Maps to OSI Transport (TCP/UDP).
3. Internet Layer: Maps to OSI Network (IP, ICMP, ARP).
4. Network Access (Link) Layer: Combines OSI Data Link and Physical layers.

4. Subnetting and Routing

4.1 IP Addressing (IPv4)

An IPv4 address is a 32-bit number divided into 4 octets (e.g., 192.168.1.1).

• Classes of IP Addresses:
  • Class A: 1.0.0.0 to 126.0.0.0 (Large networks, First octet is Net ID).
  • Class B: 128.0.0.0 to 191.255.0.0 (Medium networks).
  • Class C: 192.0.0.0 to 223.255.255.0 (Small networks/LANs).
  • Class D: Multicasting.
  • Class E: Experimental/Research.

4.2 Subnetting

The process of dividing a single large network into smaller, manageable sub-networks (subnets).

• Subnet Mask: Differentiates the Network ID from the Host ID.
  • Default Class C Mask: 255.255.255.0 (or /24 in CIDR notation).
• CIDR (Classless Inter-Domain Routing): Replaced classful addressing. Uses a suffix (e.g., /26) to indicate the number of bits used for the network portion.

Example Calculation (/26 network):

• Base: 192.168.1.0/24.
• Goal: Create 4 subnets. We need to borrow 2 bits ().
• New Mask: /26.
• Binary Mask: 11111111.11111111.11111111.11000000
• Decimal Mask: 255.255.255.192.
• Block Size (Increment): 64 ().
• Subnet 1: 192.168.1.0 to 192.168.1.63
• Subnet 2: 192.168.1.64 to 192.168.1.127

4.3 Routing

The process of selecting a path for traffic in a network.

• Static Routing: Routes are manually configured by an administrator. Secure but not scalable.
• Dynamic Routing: Routers exchange information to update routing tables automatically.
  • Distance Vector Protocols (e.g., RIP): Determine best path based on hop count. Vulnerable to routing loops.
  • Link State Protocols (e.g., OSPF): Build a full map (topology) of the network. Dijkstra’s algorithm determines the shortest path. Fast convergence.

5. Key Protocols

5.1 HTTP (HyperText Transfer Protocol) & HTTPS

• Function: Foundation of data communication for the World Wide Web. Fetches resources (HTML docs).
• Layer: Application.
• Ports: HTTP (80), HTTPS (443).
• Mechanism: Request-Response model.
  • Methods: GET (retrieve data), POST (submit data), PUT (update), DELETE.
  • Stateless: The server does not retain information about previous client requests (unless Cookies are used).
  • HTTPS: Uses SSL/TLS to encrypt the payload.

5.2 SMTP (Simple Mail Transfer Protocol)

• Function: Used for sending emails from a client to a server, or between servers.
• Layer: Application.
• Ports: 25 (legacy), 587 (modern secure submission).
• Mechanism: "Push" protocol. It pushes mail outward. It cannot "pull" mail from a server to a client.

5.3 Mail Access Protocols: POP vs. IMAP

Used by email clients (Outlook, Thunderbird) to retrieve emails from a server.

5.4 FTP (File Transfer Protocol)

• Function: Transferring files between a client and server.
• Ports:
  • Port 21: Command/Control connection (authentication, commands).
  • Port 20: Data connection (actual file transfer).
• Modes: Active and Passive (PASV is friendlier to firewalls).
• Security Note: FTP sends credentials in plain text. SFTP (SSH File Transfer Protocol) is the secure alternative.

5.5 DNS (Domain Name System)

• Function: Translates human-readable domain names (www.google.com) into IP addresses (142.250.190.46). The "Phonebook of the Internet."
• Port: 53 (UDP for queries, TCP for zone transfers).
• Hierarchy:
  1. Root Servers (.)
  2. TLD Servers (.com, .org, .edu)
  3. Authoritative Name Servers (google.com)
• Resolution Process:
  1. Client checks local cache.
  2. Asks Recursive Resolver (ISP).
  3. Resolver traverses the hierarchy (Root -> TLD -> Authoritative) to find the IP.

5.6 DHCP (Dynamic Host Configuration Protocol)

• Function: Automatically assigns IP addresses and network configuration (Subnet mask, Gateway, DNS) to devices.
• Ports: UDP 67 (Server), UDP 68 (Client).
• The DORA Process:
  1. Discover: Client broadcasts "Is there a DHCP server?"
  2. Offer: Server replies "Yes, here is an IP I can offer."
  3. Request: Client says "I accept this IP."
  4. Acknowledge: Server confirms "The IP is yours for X amount of time (Lease)."