COMP SCI UNIT 2 TEST

```

Internet

Worldwide network of connected devices. No one owns it.

World Wide Web

Web pages and files sent using HTTP. Runs ON the internet, not the same thing.

Protocol

Agreed rules for sending data.

4 Layers (top to bottom)

Application, Transport, Internet, Physical. All Teachers Ignore Phones.

Application Layer

HTTP and DNS.

Transport Layer

TCP and UDP.

Internet Layer

IP. Addresses and routing.

Physical Layer

Cables, fiber, Wi

How layers work together

Higher layers rely on the layers below them.

IP (Internet Protocol)

Gives devices addresses and routes packets.

IP Address

Unique number for each device, like a home address.

IP Address Structure

Four numbers from 0 to 255 with dots (192.168.1.1). First part is the network, last part is the device.

IPv4 vs IPv6

IPv4 is 32 bits, about 4 billion addresses, running out. IPv6 is 128 bits, practically unlimited.

TCP

Reliable but slower. Numbers packets, reorders them, asks for missing ones.

UDP

Fast but no checking. Used for streaming and gaming.

TCP vs UDP

TCP is for accuracy. UDP is for speed.

DNS

Turns names like code.org into IP addresses. The internet phone book.

How DNS scales

Many servers in a hierarchy plus caching answers, so no single server is overloaded.

HTTP

How browsers request and get web pages and files.

GET Request

Browser asks a server for a file, and the server sends it back.

How HTTP uses other layers

DNS finds the IP, TCP sends packets, IP routes them, Physical moves the bits.

HTML

Code that structures a web page.

CSS

Code that styles a web page, like colors and fonts.

CSS Syntax

selector { property: value; } for example h1 { color: blue; }

Syntax

Rules for writing code, like grammar.

ISP

Company that sells internet access.

Packets

Small chunks of a message. Each has to and from IP addresses and a number.

Router

Forwards packets toward their destination.

Dynamic Routing

Packets take different paths based on traffic or broken lines.

Redundancy

Extra backup paths and parts.

Fault Tolerance

Keeps working when parts fail. Caused by redundancy.

Scalability

Keeps working as more users join.

Bandwidth

Max amount of data sent per second.

Latency

Delay for data to arrive.

HTTPS

Encrypted HTTP. The lock icon.

SSL/TLS

Encryption that makes HTTP into HTTPS.

Why HTTPS is needed

Packets pass through many devices, so encryption keeps data private.

Certificate Authority

Trusted group that verifies a website is real.

Digital Certificate

A website's ID proving it is legit.

Encryption

Scrambling data so only people with the key can read it.

Symmetric Encryption

The same key locks and unlocks.

Public Key Encryption

The public key locks, and only the private key unlocks.

Digital Divide

Gap between people with and without tech access.

Net Neutrality

ISPs must treat all data equally.

Internet Censorship

Controlling what people can see online.

```