Application Layer Protocols; DNS — Domain Name System; DHCP — Dynamic Host Configuration Protocol

Application Layer Protocols

TELNET

TELNET: A network protocol that allows a user on one computer to log into another computer that is part of the same network.

• Establishes connection from one computer to another
• Login into a remote computer using a telnet program
• Insecure — data transmitted in plaintext
• Port: 23

telnet 192.168.1.5   # Connect to remote host

Replacement: SSH (Secure Shell) — encrypts the connection

FTP — File Transfer Protocol

FTP: An Internet tool provided by TCP/IP for transferring files between computers.

• Helps transfer files from one computer to another
• Provides access to directories/folders on remote computers
• Allows software, data, text files to be transferred
• Ports: 20 (data), 21 (control)

Active vs Passive FTP:

• Active FTP: Server initiates data connection to client

Passive FTP: Client initiates data connection (works better with firewalls)

Secure alternative: SFTP (SSH File Transfer Protocol) on port 22

URL — Uniform Resource Locator

URL: A web address — reference to a web resource specifying its location on a computer network and a mechanism for retrieving it.

https://www.example.com:443/path/page.html?query=value#anchor
|──────| |──────────────| |───| |──────────| |─────────| |──────|
Scheme      Domain        Port    Path         Query      Fragment

URL Components:

DNS — Domain Name System

What is DNS?

DNS (Domain Name System) translates human-readable domain names to IP addresses.

Why needed?

• Humans remember names (google.com) not IP addresses
• IP addresses can change (DNS allows transparent redirect)

DNS Resolution Process

Browser asks: "What is the IP of www.google.com?"

Step 1: Check local DNS cache → not found
Step 2: Check /etc/hosts file → not found
Step 3: Query Recursive DNS Resolver (usually ISP's DNS)
Step 4: Resolver queries Root DNS Server (.) 
Step 5: Root refers to TLD Server (.com)
Step 6: TLD Server refers to Google's Authoritative DNS
Step 7: Authoritative DNS returns IP: 142.250.x.x
Step 8: Resolver returns IP to browser
Step 9: Browser connects to 142.250.x.x

DNS Record Types

DNS Hierarchy

. (Root)
├── com
│   ├── google.com
│   │   ├── www.google.com
│   │   └── mail.google.com
│   └── microsoft.com
├── org
│   └── wikipedia.org
├── in
│   └── gov.in
└── edu
    └── mit.edu

DHCP — Dynamic Host Configuration Protocol

What is DHCP?

DHCP automatically assigns IP configuration to devices joining a network.

DORA Process:

D → DHCP Discover  (Client broadcasts: "Any DHCP server?")
O → DHCP Offer     (Server responds: "I offer you 192.168.1.100 for 8 hours")
R → DHCP Request   (Client: "I'll take 192.168.1.100")
A → DHCP Acknowledge (Server: "It's yours! Here's all config info")

DHCP provides:

• IP address
• Subnet mask
• Default gateway
• DNS server addresses

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