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🌐 Internet & Its Protocols

HTTP, HTTPS, FTP, SMTP, OSI model, TCP/IP, IP addressing, NAT, DNS, ports, and web protocols for UPSSSC AGTA.

What is the Internet?

The Internet is a global network of interconnected computers that communicate using standardized protocols. It originated from ARPANET (Advanced Research Projects Agency Network), created by the US Department of Defense in 1969.

Internet vs WWW: The Internet is the physical network (cables, routers, servers). The World Wide Web (WWW) is a service that runs on the Internet — a collection of web pages linked by hyperlinks. WWW was invented by Tim Berners-Lee in 1989 at CERN.

Term Meaning
Internet Global network of networks
WWW Collection of web pages (a service on Internet)
Intranet Private network within an organization
Extranet Intranet extended to authorized external users
Internet WWW intranet and extranet comparison showing global network browser based web pages private office network and partner access
The internet is the global network, the web is one service running through a browser, an intranet stays inside an organization, and an extranet gives limited outside access to trusted partners.

OSI Model — 7 Layers

The OSI (Open Systems Interconnection) model divides network communication into 7 layers. It was developed by ISO (International Organization for Standardization).

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

Layer Name Function Protocol/Device
7 Application User interface, email, browsing HTTP, FTP, SMTP, DNS
6 Presentation Data formatting, encryption, compression SSL/TLS, JPEG, MPEG
5 Session Establishes/manages sessions NetBIOS, RPC
4 Transport Reliable data delivery, error checking TCP, UDP
3 Network Routing, IP addressing IP, ICMP, Router
2 Data Link Frame delivery, MAC address Ethernet, Switch, Bridge
1 Physical Bits over cables, signals Hub, Repeater, Cables

Remember: Data flows down at sender (Application → Physical) and up at receiver (Physical → Application).

OSI model seven layers showing sender downward flow and receiver upward flow for internet protocols
The OSI model is easiest to remember as seven layers that data descends through at the sender and ascends through at the receiver.

TCP/IP Model — 4 Layers

The TCP/IP model is the practical model used by the Internet. It has 4 layers:

TCP/IP Layer OSI Equivalent Function
Application Application + Presentation + Session HTTP, FTP, SMTP, DNS
Transport Transport TCP (reliable), UDP (fast)
Internet Network IP addressing, routing
Network Access Data Link + Physical Physical transmission

TCP vs UDP

TCP versus UDP showing acknowledgments retransmission and reliable delivery compared with faster no acknowledgement packet flow
TCP prioritizes reliable ordered delivery with acknowledgments and retransmission, while UDP sacrifices that overhead for faster delivery in streaming, gaming, and voice traffic.
Feature TCP UDP
Full Form Transmission Control Protocol User Datagram Protocol
Connection Connection-oriented Connectionless
Reliability Reliable (acknowledgment) Unreliable (no ack)
Speed Slower Faster
Use Web, email, file transfer Video streaming, gaming, DNS
Example HTTP, FTP, SMTP VoIP, live video, DHCP

Important Protocols & Port Numbers

Protocol Full Form Port Purpose
HTTP HyperText Transfer Protocol 80 Web page access (unencrypted)
HTTPS HTTP Secure 443 Encrypted web (SSL/TLS)
FTP File Transfer Protocol 20 (data), 21 (control) File upload/download
PPP Point-to-Point Protocol — Carries TCP/IP traffic over direct serial links such as telephone connections
SMTP Simple Mail Transfer Protocol 25 Sending emails
POP3 Post Office Protocol 3 110 Receiving email (downloads)
IMAP Internet Message Access Protocol 143 Receiving email (server sync)
TELNET Telecommunication Network 23 Remote access (unencrypted)
SSH Secure Shell 22 Secure remote access
DNS Domain Name System 53 Domain → IP resolution
DHCP Dynamic Host Configuration Protocol 67/68 Auto-assigns IP addresses
SNMP Simple Network Management Protocol 161 Network device monitoring

HTTPS = HTTP + SSL/TLS encryption. Banks and login pages always use HTTPS (look for the lock icon in the browser).

An older but important exam term is PPP (Point-to-Point Protocol). It is a data-link-layer protocol used to carry network traffic over a direct link, especially in dial-up or telephone-line style serial connections.

IP Addressing

An IP address uniquely identifies every device on a network.

IPv4

IPv4 uses 32 bits written as 4 octets separated by dots (e.g., 192.168.1.1).

Class Range Default Subnet Mask Use
A 1.0.0.0 – 126.255.255.255 255.0.0.0 Large networks
B 128.0.0.0 – 191.255.255.255 255.255.0.0 Medium networks
C 192.0.0.0 – 223.255.255.255 255.255.255.0 Small networks (most common)
D 224.0.0.0 – 239.255.255.255 — Multicasting
E 240.0.0.0 – 255.255.255.255 — Research/Experimental
  • 127.0.0.1 = Loopback address (localhost) — device refers to itself
  • Total IPv4 addresses: ~4.3 billion (2³²)

IPv6

IPv6 uses 128 bits written in 8 groups of hexadecimal (e.g., 2001:0db8:85a3::8a2e).

  • Total addresses: 2¹²⁸ (virtually unlimited)
  • No need for NAT
  • Built-in security (IPSec)

NAT (Network Address Translation)

NAT allows multiple devices on a private network to share a single public IP address to access the Internet.

  • Why needed: IPv4 addresses are limited; NAT conserves public IPs
  • How it works: Router translates private IPs (192.168.x.x) to a public IP for internet communication
  • Types: Static NAT (1:1 mapping), Dynamic NAT (pool of public IPs), PAT/NAT Overload (many-to-one using port numbers — most common)
NAT router translating private IP addresses to one public IP address for internet access
NAT lets several private-network devices share one public IP address when communicating with the internet.

URL Structure

A URL (Uniform Resource Locator) is the address of a web resource:

https://www.example.com:443/path/page.html?id=1#section
  |        |          |    |              |      |
Protocol  Domain     Port  Path         Query  Fragment

A website's home page is the main or starting page that usually opens first when you visit the domain. In exam language, "home page" means the default introductory page of a website, while a URL is the full address used to reach a specific page or resource.

Web Browsers & Search Engines

Web Browsers Search Engines
Google Chrome Google
Mozilla Firefox Bing (Microsoft)
Safari (Apple) Yahoo
Microsoft Edge DuckDuckGo
Opera Baidu (China)

A browser displays web pages; a search engine finds them. Google is a search engine accessed through a browser.

History of the Internet — Key Facts

Fact Details
ARPANET First Message Sent from UCLA to Stanford on Oct 29, 1969 — only "LO" was transmitted (system crashed before completing "LOGIN")
WWW Inventor Tim Berners-Lee invented WWW at CERN (Switzerland) in 1989; first website: info.cern.ch
First Web Browser The first browser created by Tim Berners-Lee was called WorldWideWeb, later renamed Nexus
Father of Internet Vint Cerf and Bob Kahn — designed the TCP/IP protocol
ICANN Internet Corporation for Assigned Names and Numbers — manages domain names globally
ISP Internet Service Provider — provides internet access (e.g., Jio, Airtel, BSNL in India)

Bandwidth vs Latency

Term Meaning Analogy
Bandwidth Maximum data capacity of a connection Width of a water pipe
Latency Time delay for data to travel from source to destination Time for water to flow through the pipe

High bandwidth + low latency = best internet experience. Bandwidth is measured in bps (bits per second); latency in ms (milliseconds).

Protocol Transport — TCP vs UDP Usage

Uses TCP (Reliable) Uses UDP (Fast)
HTTP, HTTPS DNS
FTP DHCP
SMTP SNMP
SSH TFTP
TELNET VoIP (Voice over IP)

Port Number Ranges

Range Name Description
0 – 1023 Well-known ports Reserved for standard services (HTTP=80, FTP=21, SSH=22)
1024 – 49151 Registered ports Used by specific applications (MySQL=3306, PostgreSQL=5432)
49152 – 65535 Dynamic/Private ports Temporary ports assigned by OS for client connections

Subnet Mask & Private IP Ranges

A subnet mask determines which part of an IP address is the network portion and which is the host portion.

  • Example: 255.255.255.0 (Class C) means first 3 octets = network, last octet = host
  • Used by routers to decide if traffic stays local or goes to another network

Private IP Address Ranges (Not Routable on Internet)

Class Private IP Range Subnet Mask No. of Addresses
A 10.0.0.0 – 10.255.255.255 255.0.0.0 ~16.7 million
B 172.16.0.0 – 172.31.255.255 255.255.0.0 ~1 million
C 192.168.0.0 – 192.168.255.255 255.255.255.0 65,536

Your home WiFi router typically uses 192.168.x.x (Class C private range). These addresses are free to use internally but need NAT to access the internet.

Summary Points

Concept Key Details
ARPANET First internet (1969), US DoD
ARPANET First Message UCLA → Stanford, Oct 29, 1969 — only "LO" sent (crashed before "LOGIN")
WWW Tim Berners-Lee, 1989, CERN (Switzerland)
First Website info.cern.ch
Father of Internet Vint Cerf & Bob Kahn — designed TCP/IP
ICANN Manages domain names globally
ISP Internet Service Provider (Jio, Airtel, BSNL)
Internet vs WWW Internet = physical network; WWW = web pages (service on Internet)
Intranet / Extranet Intranet = private org network; Extranet = intranet + authorized external users
OSI Model 7 layers — Application to Physical (mnemonic: APSTNDP)
TCP/IP Model 4 layers — Application, Transport, Internet, Network Access
TCP Reliable, connection-oriented, slower — HTTP, FTP, SMTP, SSH, TELNET
UDP Fast, connectionless, unreliable — DNS, DHCP, SNMP, TFTP, VoIP
HTTP/HTTPS Port 80/443, web access
FTP Port 20 (data) / 21 (control), file transfer
SMTP Port 25, sending email
POP3 / IMAP Port 110 / 143, receiving email (download vs server sync)
SSH Port 22, secure remote access
TELNET Port 23, unencrypted remote
DNS Port 53, domain → IP resolution
DHCP Port 67/68, auto-assigns IP addresses
SNMP Port 161, network device monitoring
Well-known Ports 0–1023 — reserved standard services (HTTP=80, SSH=22)
Registered Ports 1024–49151 — specific apps (MySQL=3306)
Dynamic Ports 49152–65535 — temporary OS-assigned client ports
IPv4 32-bit, 4 octets, ~4.3 billion (2^32)
IPv6 128-bit, 8 hex groups, unlimited, built-in IPSec
Class A 1–126.x.x.x, large networks, mask 255.0.0.0
Class B 128–191.x.x.x, medium networks, mask 255.255.0.0
Class C 192–223.x.x.x, small/most common, mask 255.255.255.0
Class D / E D = multicasting (224–239); E = experimental (240–255)
127.0.0.1 Loopback / localhost
Private IP: Class A 10.0.0.0 – 10.255.255.255 (~16.7M addresses)
Private IP: Class B 172.16.0.0 – 172.31.255.255 (~1M addresses)
Private IP: Class C 192.168.0.0 – 192.168.255.255 (65,536 addresses)
Subnet Mask Determines network vs host portion (e.g., 255.255.255.0)
NAT Private → Public IP translation
PAT / NAT Overload Many-to-one NAT using port numbers — most common type
Bandwidth Max data capacity of connection (measured in bps) — like pipe width
Latency Time delay source → destination (measured in ms) — like water travel time
SSL/TLS Encryption for HTTPS
URL Structure Protocol://Domain:Port/Path?Query#Fragment

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