Internet & Its Protocols

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.

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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

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

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TCP/IP Model — 4 Layers

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

TCP vs UDP

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Important Protocols & Port Numbers

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

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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).

• 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)

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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)

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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

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Web Browsers & Search Engines

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

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History of the Internet — Key Facts

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Bandwidth vs Latency

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

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Protocol Transport — TCP vs UDP Usage

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Port Number Ranges

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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)

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.

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Key Takeaways

• ARPANET first message: UCLA → Stanford, Oct 29, 1969 — only “LO” transmitted (crashed before “LOGIN”)
• WWW invented by Tim Berners-Lee at CERN (1989); first website: info.cern.ch
• Father of Internet: Vint Cerf & Bob Kahn — designed TCP/IP protocol
• OSI has 7 layers (mnemonic: All People Seem To Need Data Processing); TCP/IP has 4 layers
• TCP = reliable + slow (HTTP, FTP, SMTP, SSH); UDP = unreliable + fast (DNS, DHCP, VoIP, SNMP)
• HTTP (port 80), HTTPS (443), FTP (20/21), SMTP (25), SSH (22), TELNET (23), DNS (53)
• Port ranges: Well-known (0-1023), Registered (1024-49151), Dynamic/Private (49152-65535)
• IPv4 = 32-bit, ~4.3 billion addresses; IPv6 = 128-bit, unlimited, built-in IPSec
• Private IP ranges: Class A (10.x.x.x), Class B (172.16-31.x.x), Class C (192.168.x.x)
• Subnet mask determines network vs host portion of IP (e.g., 255.255.255.0 for Class C)
• NAT converts private IPs to public IPs; PAT/NAT Overload = many-to-one using port numbers
• Bandwidth = max data capacity (bps); Latency = time delay (ms) — high BW + low latency = best
• ICANN manages domain names globally; ISP provides internet access
• 127.0.0.1 = localhost (loopback); URL structure: Protocol://Domain:Port/Path?Query#Fragment

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Summary Cheat Sheet