3.Network, Internet, WWW - YukaKoshiba/MyknowledgeDocs GitHub Wiki

Network/Internet/WWW @English Version
Create Date：2025/07/09
Last Update Date：2025/07/09

Network
Network identifiers： IP address　 MAC address
[Connection methods] Packet
[Connection rules] Protocol： The OSI(Open Systems Interconnection) model ( DHCP )
[Network devices] Network switch　 Router　 Access Point　

[Network Type] LAN　 WAN　 Internet

Internet
[How the Internet works] WWW(Web)
[Companies providing Internet] ISP
[Protocol(rules)] TCP/IP protocol suite　 Explanation of each protocol ( Port Number )
HTTP request methods(GET/POST)　 HTTP status codes　 Cookies and Sessions
[Components] DNS Server

Web Applications：
Web Browser　 Search Engine　 Search Options　 Web site ( Static website　 Dynamic website )
Webpage ( URL　 Domain　 DNS Server )　 Developer Mode (Developer Tools)

A network is the most fundamental concept in computing:
It's a system where multiple computers and devices are connected to each other, enabling them to exchange data.
It's a technology (mechanism) designed to connect multiple computers and devices to facilitate information exchange.

Its functions include:
・Sharing resources
・Facilitating communication

Networks are classified into various types, such as LANs (Local Area Networks) and WANs (Wide Area Networks), depending on their scale and connection methods.
Examples: Networks vary greatly in size, including home LANs, company intranets, and the internet.

(As a side note: 95% of network troubles are caused by cables.)

An Internet Protocol (IP) address is a unique identification number assigned to each device on a network, based on the Internet Protocol.
It acts like a street address on a network, used to specify the source and destination for sending and receiving data.

Types of IP Addresses
There are two main types of IP addresses: Global IP addresses and Private IP addresses.br> ・Global IP Address: An IP address that is uniquely identified on the internet.
・Private IP Address: An IP address used within a local network (like your home or office network).

IP Address Notation
IP addresses are written in different formats depending on their version:
・IPv4: A 32-bit decimal format (e.g., 192.168.1.1).
・IPv6: A 128-bit hexadecimal format (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334).
The number of devices that can be accommodated by IPv4 is no longer sufficient.
In recent years, there's been a growing movement to transition from IPv4 to IPv6.

A unique physical identification number specific to a network device.
Embedded in the Network Interface Card (NIC) and typically cannot be changed.
Used to identify a device within a local network.

Packet In networking, a packet is the fundamental unit of communication.
It's a small, self-contained unit of data used to efficiently send and receive information over a network.
Packets are utilized not only on the internet but also in local area networks (LANs) and other network types.

Role of Packets：
・Efficient Data Transmission: By breaking large amounts of data into smaller packets, they help avoid network congestion and enable more efficient data transfer.
・Error Control: Each packet can include error detection and retransmission mechanisms, which significantly increases the reliability of data transmission.
・Enabling Diverse Communications: Dividing different types of data into packets allows networks to support various communication protocols.

Packet Structure and Standardization
Packets are standardized units. Each fragmented packet typically includes:
・Header: Contains essential information like the sender's and receiver's addresses, as well as other control data.

・Payload: This is the actual data you want to send.

Example: IP Packet on the Internet
An IP packet is a common example of a packet used on the internet.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| IHL |Type of Service| Total Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification |Flags| Fragment Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time to Live | Protocol | Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Protocol A protocol is a set of rules or etiquette for communication used in a network.
Think of it as the communication rules that allow two completely unfamiliar computers to talk to each other.
Protocols define various aspects of communication, including data format, transmission and reception procedures, and error handling.

The OSI Model is a framework standardized by ISO (International Organization for Standardization) that defines the mechanics of network communication by dividing it into seven distinct layers.
Each layer has a different role, and the model aims to enable intercommunication between devices from different manufacturers by clearly defining the interfaces between layers.
t's important to remember that the OSI basic reference model is purely a conceptual model and is not directly used in actual networks.
For example, the internet primarily uses the TCP/IP protocol suite, which is based on the principles of the OSI model.

DHCP is a protocol (set of rules) for automatically assigning network configurations, such as IP addresses, within a network.
It automatically assigns information like IP addresses, subnet masks, default gateways, and DNS server addresses to computers and devices connecting to the network.

This means network administrators no longer need to manually configure IP addresses for each device, making network setup and management more efficient.

Initial Configuration
For the initial setup, since the source IP address is unassigned, it's set to 0.0.0.0.
The destination IP address is set to 255.255.255.255, and a broadcast is sent to the DNS Server.

Before DHCP existed, you had to manually configure IP addresses on each device by looking at a piece of paper provided by your Internet Service Provider (ISP) when you signed up for internet service.
This was known as static IP address configuration.
Now, with DHCP, this process is automated by the DNS Server, making internet connectivity for devices much easier and more efficient.

In modern networks, routers, access points, and switches are often integrated into a single device, making it less common to consider them separately.

A router is a device that relays data between different networks (for example, between your home LAN and the internet).
It enables multiple devices to connect to the internet simultaneously and ensures data is sent and received correctly across different networks.
It functions as both a network terminal and an internet terminal.

Routers use routing tables to forward data along the most appropriate path based on IP addresses.
Additionally, a router can select the optimal route based on network conditions, thereby improving data transmission efficiency (much like a traffic controller).

An Access Point is a device primarily used in wireless LAN (Wi-Fi) environments to connect devices to a network wirelessly.
It acts as the internet selected by the client.

It's important to note that an Access Point itself does not have internet connectivity.
Its sole role is to bridge wireless-enabled devices like smartphones and computers with a wired LAN.

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The Internet is a global network that connects computers worldwide.
It's simply a massive collection of interconnected networks designed for computers to exchange information.
You can think of it as the infrastructure—the electronic web or wires—that allows billions of computers to connect.

It's important to note that the Internet (infrastructure) ≠ WWW: the World Wide Web (software).
The Internet is the technical infrastructure that connects all the billions of computers providing the WWW.
In essence, it's the system that links multiple computers and devices for information exchange.

The ARPANET is considered the progenitor of the Internet, serving as its prototype.
Developed in the late 1960s by the U.S. Department of Defense's Advanced Research Projects Agency (ARPA), it was the world's first computer network to use packet switching.
Initially, it was designed for military purposes, aiming to build a decentralized communication network.

All communications over the Internet operate based on common protocols (communication rules).
Therefore, the Internet can be described as a protocol system.
The most crucial protocols on the Internet are TCP/IP (Transmission Control Protocol/Internet Protocol).

The World Wide Web (WWW), often simply called "the Web," is a hypertext system provided over the internet.
It's a global information space accessible from anywhere in the world.
Essentially, it's a system that interconnects information on the internet using links, allowing users to view it via a web browser.

Important Distinctions:
・WWW (the World Wide Web) is a software system, not the internet itself.
The internet is the underlying infrastructure, while the WWW is a service built upon it. Think of the internet as the roads and highways, and the WWW as the collection of houses and businesses built along those roads, interconnected by pathways (links).
・The WWW is not the same as "www" in a URL.
While many website addresses start with "www" (e.g., www.example.com), this is merely a common subdomain convention and not synonymous with the entire World Wide Web system.

An ISP (Internet Service Provider) is a company that provides internet services.

The TCP/IP protocol suite refers to a standard set of communication protocols used on the internet.
OIt's structured with four layers, which are simpler than the seven layers of the OSI model.
This suite is widely used in actual internet communication because it evolved into a more practical form, drawing concepts from the OSI basic reference model.

The OSI Model	TCP/IP	Implementation Examples
Application Layer Presentation Layer Session Layer	Application Layer	HTTP,SMTP,FTP,DHCP, DNS,SNMP,POP3,Telnet
Transport Layer	Transport Layer	TCP,UDP
Network Layer	Internet Layer	IP,ICMP
Data Link Layer Physical Layer	Network Interface Layer Ethernet, etc.	Ethernetなど

The most crucial protocols on the internet are often referred to as TCP/IP (Transmission Control Protocol/Internet Protocol).
However, it's important to understand that TCP/IP is a combination of two distinct protocols: TCP and IP.
It doesn't refer to a single, standalone protocol.

TCP/IP protocol suite The OSI Model	Protocol Name	Role
Application Layer Application Layer	HTTP Hypertext Transfer Protocol	acilitates the exchange of information, such as web pages, between web browsers and web servers. HTTP is used when you enter a URL in a web browser to display a website, or when sending/receiving data via Web APIs. It defines the format for web page requests and responses, including: • How a web browser requests a web page from a web server. • How a web server returns the requested information to a web browser.
Application Layer Application Layer	HTTPS Hypertext Transfer Protocol Secure	An extension of HTTP that encrypts communication using SSL/TLS to enhance security.
Transport Layer Transport Layer	TCP(Transmission Control Protocol) Transmission Control Protocol	Enhances the reliability of communication where IP alone is insufficient. It provides the following functionalities to ensure data is reliably delivered to the recipient: • Connection-oriented communication: 　Establishes a connection with the communication partner before sending/receiving data. • Sequence control: Ensures packets arrive in the correct order. • Retransmission control: Requests retransmission if a packet fails to arrive. • Error control: Detects and corrects data errors. These features make TCP essential for applications requiring highly reliable data transmission (e.g., web browsers, email). By combining IP addresses with port numbers, TCP ensures data is correctly delivered to the appropriate application.
Internet Layer Network Layer	IP Internet Protocol	The fundamental protocol for sending and receiving data over the internet. It divides data into small units called packets, assigns destination information (IP addresses) to each packet, and sends them. IP is a connectionless protocol, meaning it transmits data without establishing a prior connection with the communication partner. While it's responsible for delivering packets to their destination, it does not guarantee that packets will arrive reliably or in the correct order—that role is handled by TCP.

Port numbers are used to identify specific services running on a device.
Think of an IP address as the address for a computer, and a port number as the specific number identifying the application running on that computer.
TCP delivers data to the correct application by combining an IP address and a port number.

Port numbers range from 0 to 65535 and are broadly divided into three categories:

• Well-known ports(0～1023)
  Assigned to common services (e.g., Web, email).
• Registered Ports (1024–49151)
  Used by specific applications.
• Dynamic/Private Ports (49152–65535)
  Temporarily used by client-side applications.

Well-known ports

Port Number	Service	Protocol
80	HTTP	TCP
443	HTTPS	TCP
20:For Transfer 21:For Control	FTP	TCP
25	SMTP	TCP
110	POP3	TCP
53	DNS	TCP/UDP
123	NTP	UDP

　NTP 123/UDP

An HTTP request follows a format similar to this:

GET / HTTP/2 Host: www.harvard.edu

This requests the host server at www.harvard.edu to retrieve information using the GET method with HTTP Version 2.

HTTP Request Methods (GET/POST) These are the request methods used in the communication protocol for exchanging data between a web browser and a web server over HTTP or HTTPS.
They define how data is retrieved or sent using HTTP/HTTPS.

HTTP request methods Methods Description GET Used to retrieve data from a web server.
Parameters are sent to the server within the URL.
Expected not to modify data on the server.
May remain in browser history and cache.
Not suitable for sending sensitive data. POST Used to send data to a web server.
Parameters are sent to the server within the request body.
May modify data on the server.
Expected not to remain in browser history or cache.
Suitable for sending sensitive data.

Comparing GET and POST
If we were to use an analogy to explain the difference between GET and POST:

GET：Like a Postcard
The destination (URL) and the content (parameters) are directly written on the surface.
The post office worker (server) looks at the destination and content to deliver it (retrieve data).
Since anyone can see the content, it's not suitable for sending highly confidential information.

POST: Like a Letter or Package in an Envelope
Only the destination (URL) is written, and the content (parameters) are inside the envelope.
The post office worker (server) looks at the destination and content to deliver it (send/process data).
Since the content can't be seen by others, it's suitable for sending highly confidential information.

Methods for Specifying HTTP Request Methods
There are various ways to specify HTTP request methods like GET and POST:

1.HTML Forms: By using the method attribute within an HTML <form> tag.
2.JavaScript: Within your JavaScript code, you can specify the method value using objects like XMLHttpRequest or the Fetch API.
3.Command-Line Tools: By utilizing tools such as curlto specify the method.
4.Hyperlinks: When you use an <a> (hyperlink) tag, it will always result in a GETrequest.

An HTTP response typically follows this format:

HTTP/2 200 OK Content-Type: text/html

The three-digit number following the HTTP version is called the HTTP status code.
This code indicates the status of the server's response, showing whether the request was successful or if any issues occurred.

Modern web browsers come with built-in developer tools that allow you to inspect the details of HTTP requests and responses.
Using these tools, you can check status codes, header information, and more, to analyze how a web page is functioning.

Code	Status	Meaning
200	OK	リクエスト成功(レスポンスが正常に返された)
301	Moved Permanently	Request successful (response returned normally)/td>
302	Found	Page has permanently moved to a different URL
401	Unauthorized	Authentication is required for the request
404	Not Found	The requested resource was not found
500	Internal Server Error	A server-side error occurred
503	Service Unavailable	The server is temporarily unavailable
504	Gateway Timeout	The server timed out

Modern web browsers come with built-in developer tools that allow you to inspect the details of HTTP requests and responses.
By using these tools, you can check status codes, header information, and other details to analyze how a webpage functions.

Cookies and sessions are crucial mechanisms for maintaining user state and information over HTTP, which is a stateless protocol.

HTTP's Stateless Nature:
HTTP is a stateless protocol, meaning each client (web browser) request and server response is a one-to-one, independent interaction.
The server does not remember past requests from a client.

In web applications, however, it's essential to maintain a user's state across a series of operations, such as keeping users logged in, retaining shopping cart contents, or tracking site activity.
Cookies and sessions are used to achieve this state management on top of the stateless HTTP.

Cookie
Cookie are small pieces of text data stored on the client-side (your web browser).
A web server sends a cookie to your browser, which then saves it locally.
Subsequently, when your browser sends another request to the same web server, it includes the saved cookie in the HTTP request header.
This allows your computer to tell the server that you're an authenticated user who has logged in before, helping the server remember previous communications, such as your login status.
Note: If you delete cookies along with your Browse history, you'll be logged out of websites.

Sessions
Sessions are a mechanism for temporarily storing user-specific information on the server-side.
The series of operations a client (web browser) performs from accessing a website until leaving it is called a session.
During this session, the server manages the user's state and information.

Applications that run over the internet and can be accessed and used through a web browser.

Software used to access the internet.
It receives and displays webpages.
Its primary role is to receive files like HTML, CSS, and JavaScript sent from a web server, parse them, and display them in a user-friendly format.
It acts as the "client" for running web applications.

A website that helps users find webpages from other websites.
It's important not to confuse infrastructure (like web browsers) with services (like search engines).

Search Method	What it Does
"search term"	Searches only for sites that contain that exact keyword or phrase. Useful when you need to search for a specific, complete phrase.
search term1 OR/AND search term2	Searches for pages containing either search term1 or search term2 (OR), or pages containing both search term1 and search term2 (AND).
**search phrase with part of it including ***	Performs a fuzzy search (or wildcard search). This finds variations or partial matches of your phrase.
related:URL	Finds websites related to the specified URL.
search term not excluded term	Excludes results based on context. This is often used for more nuanced exclusion than the hyphen method.
search term -excluded term1 -excluded term2	Excludes specific words or phrases from your search results. Useful when you want to filter out particular terms.
site:specific_site_URL keyword)	Searches only within a specific website for your keywords.
cache:URL	Allows you to view a cached version of a webpage stored by Google, even if the original site is currently unavailable.
link:URL	Shows you which sites link to the specified URL.
filetype:extension search_keyword	Searches for files matching the specified extension that are related to your search keyword.
weather:prefecture_name/city_name	Searches for the weather forecast for the specified prefecture or city.

Websites are information services provided over the internet.

They are a collection of webpages on the World Wide Web (WWW), organized around a specific theme or purpose.
Examples include corporate homepages, personal blogs, and online stores.

Storage Location: Web servers
　 Owners: Web owners (ownership is decentralized; no single entity owns the entire internet)
Address: IP address

There are fundamental differences in how website content is displayed to users and how it's generated/managed.
Websites can be categorized into Static Websites and Dynamic Websites.

A static website is a website where content such as HTML, CSS, and JavaScript files are pre-created and stored on a web server before a user accesses them.
This means that no matter who accesses the site, when, or from where (e.g., a company's homepage), the exact same content is always displayed.

Characteristics:
・Fast page loading: Pages load quickly because there's minimal server-side processing.
・Lower security risk: Security risks are relatively low as there's no database interaction or server-side program execution.
・Stable server load: Server load remains relatively stable, even with increased access.
・Manual content updates: Updating content requires directly editing HTML files and re-uploading them to the server, which can be time-consuming.

A dynamic website is a type of website where programs are executed on the web server when a user accesses a webpage.
Based on the results of these programs, HTML and other content are generated in real time.
This means the content is created in response to the user's actions and behavior, and then sent to their browser as HTML.

Characteristics:
・The displayed content changes based on factors like the access time, location, and the user's operation history.
・By integrating with a database, the latest information is always displayed.
・Utilizing a CMS (Content Management System) allows for relatively easy addition, editing, and management of content without requiring specialized knowledge.
・Since server-side processing is necessary, page loading times can sometimes be longer compared to static sites.
・Due to the presence of databases and server-side programs, robust security measures are crucial.
・Server load can become high depending on the number of accesses and the complexity of processing.
・Requires knowledge of server-side languages and databases, in addition to the HTML, CSS, and JavaScript that constitute static sites.

A single unit of information that makes up a website.
It's composed of HTML, CSS, and JavaScript.

A URL is the unique address of a resource published on the internet, such as an HTML page, CSS document, or image.

Generally, the forward slash (/) after the domain in a URL indicates a directory (folder).

A URL ending with a slash, such as http://example.com/directory/ is interpreted by the web server as a request for the default file within the "directory" (usually index.html, index.php, or whatever is configured on the server). If the server successfully finds and responds with the default file, the Status Code will be 200 OK.

If a URL like http://example.com/directory (without a trailing slash) refers to an actual directory, the web server might behave in the following ways, depending on its configuration:

・Redirect (301 Moved Permanently or 302 Found)
The server might determine that this URL refers to a directory and needs to be redirected to the correct URL ending with a slash (http://example.com/directory/).
It then responds with a 301 (permanent redirect) or 302(temporary redirect)Status Code, including the correct URL in the Locationheader.
The browser receives this and automatically makes a new request to the URL with the trailing slash.

・Serve Default File (200 OK)
In some server configurations, if a directory is specified with a URL without a trailing slash, the server might automatically locate and serve the default file within that directory, responding with a 200 OK.
In this case, the displayed webpage will look the same as with a trailing slash, but the internal processing differs.

・404 Not Found
Due to server misconfiguration or if a default file doesn't exist for the directory when accessed via a URL without a trailing slash, the server might return a 404 Not Found error.

A string added to the end of a URL in the format ?key=value, used to send additional information to a web server.

?: Separates the URL's path from the beginning of the query string.
key: A string representing the parameter's name, used by the server to identify what information was received.
=: A symbol that links the key to its corresponding value.
value: The specific data that corresponds to the key.
&: Used to connect multiple key=value pairs.

A part of a URL, serving as a human-readable address.
It helps you access websites without having to remember their IP addresses.

通常、Domainはwwwを除いた部分を示す
一般的にwwwはホスト名と呼ばれ、会社の特定のサーバの名前のようなもの
＝独自にホストサーバを運営している組織は、"www"ではない場合もある
(例：MITのURL https://web.mit.edu/)
また、Domainの最後の".〜"の部分は、TDL(Top Domain Level)と呼ばれ、どの様な種類のDomainであるかを示す指標

ICANN(Internet Corporation for Assigned Names and Numbers)という非営利団体がTLDを管理し、新しいTLDの導入や既存のTLDの管理を行っている

メリット：
・インターネット上の膨大な数のウェブサイトを効率的に管理できる
・ セキュリティと信頼性の向上
・企業は、自社のブランド名に合わせたTLD(ブランドTLD)を取得することで、ブランドの保護や識別を強化できる

TDL	意味
.com	一般的なDomain 通常商用利用される
.edu	教育機関
.org	非営利団体
.gov	政府機関
.us/.jp/.cnなど	国ごとのDomain

A DNS server is like a dictionary that manages the mapping information (or association) between IP addresses and domain names.

Thanks to DNS servers, you don't need to memorize complex IP addresses. Instead, you can simply type a human-friendly domain name to access your desired webpage.

Types of DNS Servers

Type of DNS Server	Role
Authoritative DNS Server	This server holds the ultimate authority for information related to a specific domain name. The domain's administrator configures information such as the mapping between the domain name and its IP address on this server.
Caching DNS Server	This server checks and resolves domain name information. It stores (caches) domain name information it has queried once for a certain period, allowing for quicker responses to subsequent queries.

Developer Mode (also known as Developer Tools) is a set of features designed for web developers to analyze and debug the structure, styles, and behavior of web pages.
In recent years, its use has expanded beyond web developers to general users who want to understand a website's structure or examine its behavior in detail.

How to Open Developer Tools:
You can open Developer Tools using one of these methods: ・Press F12.
・PressCtrl + Shift + c.
・Right-click anywhere on a webpage and select "Inspect" (or "Inspect Element").

What You Can Do with Developer Tools:
・Inspect and Temporarily Edit HTML and CSS: View and make temporary changes to a web page's HTML structure and CSS styles directly in your browser.
・Debug JavaScript: Identify and fix errors in your JavaScript code.
・Monitor Network Requests: Analyze the headers, content, and timing of requests and responses to and from the server.
・Analyze Performance: Measure page load times and rendering performance to identify and optimize bottlenecks.
・Manage Storage: View and edit web storage content, including cookies, local storage, and session storage.
・Check Mobile Display: Simulate how a page looks on different screen sizes and devices, which is helpful for testing responsive design.
・Verify Accessibility: Simulate how screen readers interpret a page to ensure it's accessible to all users.

Official Documents
Google Chrome ( English / Japanese)

English / <a href="https://learn.microsoft.com/ja-jp/microsoft-edge/devtools-guide-chromium/overview"Japanese)

Tabs	Imformation
Elements	A panel where you can inspect and temporarily edit the HTML content of the webpage you're viewing.
Console	A panel to view detailed information about resources loaded by the browser, such as HTML files. You can also perform JavaScript step-by-step execution to debug and understand code behavior in detail.
Network	A panel to inspect the communication logs (HTTP requests/responses) made by the browser.
Performance	A feature to thoroughly investigate factors causing slow page loading and those impacting Core Web Vitals metrics.
Application	A panel where you can view, edit, and delete browser-held data like Cookies and localStorage that might be exchanged within the page.
Lighthouse	A panel that allows you to perform quality assessment tests on web pages, including those not publicly accessible.
Recorder	A panel where you can record, replay, and analyze the performance of user interactions (like clicks and input) on a Chrome page.

You can select elements on the webpage to inspect their corresponding HTML and CSS.
The select mode is active when the icon is blue.

You can temporarily edit HTML and CSS in the Elements tab.
The webpage displayed to the client is a copy loaded from the web server, so any edits you make won't be reflected back on the original web server.
If you want to revert your temporary edits, simply reload the page to restore it to its original state.

Activating Device Mode reduces the browser's viewport width, simulating how a webpage appears on a smartphone.
Tapping the Device Mode icon once more will revert the display to its standard PC size.
You can select a unique device name from the device selection menu to test how content appears on each device.
If your desired device isn't available in the menu, you can also add a new one.

You can also change the device's screen orientation between portrait and landscape modes, just like a real smartphone, to check how the display looks.

Word	Meaning, Notes
Accessibility	The ability of a product, building, or service to be easily used and understood by everyone, including people with disabilities and the elderly.
Visual Accessibility	Refers to the ease with which visual elements can be perceived and understood. Contrast between elements is crucial. For example, the contrast ratio between text and background in headings should be at least 4.5:1.
Screen reader	Software that converts digital text and images into speech, braille, or other non-visual formats, allowing people with visual impairments to access information displayed on a screen.

GeeksforGeeks
mdn web docs
ProEngineer
W3CScholl