📚 Computer Networks and the Internet | 计算机网络与互联网
A computer network is a group of interconnected devices that can share data and resources. Understanding how networks operate is essential for modern computing, as nearly every digital service relies on communication between devices.
计算机网络是一组能够共享数据和资源的互连设备。理解网络的运行方式对现代计算至关重要，因为几乎所有数字服务都依赖设备之间的通信。
1. What Is a Computer Network? | 什么是计算机网络？
A computer network is formed when two or more computing devices are linked together to exchange data. The devices can include computers, smartphones, printers, servers and many other hardware components.
当两台或更多计算设备连接起来交换数据时，就形成了计算机网络。这些设备可以包括计算机、智能手机、打印机、服务器和许多其他硬件组件。
Networks allow users to share files, use shared peripherals, communicate through email and access centralised applications. Without networks, each device would operate in isolation and data transfer would require physical media such as USB drives.
网络使用户能够共享文件、使用共享外设、通过电子邮件交流以及访问集中式应用程序。如果没有网络，每台设备都将孤立运行，数据传输就需要借助U盘等物理介质。
2. Types of Networks: LAN, WAN and PAN | 网络类型：局域网、广域网和个域网
A Local Area Network, or LAN, covers a small geographical area such as a home, school or office. LANs are usually privately owned and offer high data transfer speeds because the distances are short.
局域网（LAN）覆盖家庭、学校或办公室等较小的地理区域。局域网通常为私人所有，由于距离短，数据传输速度较高。
A Wide Area Network, or WAN, spans a large geographical area, often a country or continent. The internet is the largest example of a WAN, connecting millions of LANs across the globe.
广域网（WAN）跨越较大的地理区域，通常是一个国家或大洲。互联网就是最大的广域网，连接了全球数百万个局域网。
A Personal Area Network, or PAN, is centred around a single person and their immediate devices, such as a smartphone connected to wireless earphones or a laptop linked to a smartwatch via Bluetooth.
个域网（PAN）围绕一个人及其近距离设备构建，例如智能手机连接无线耳机，或笔记本电脑通过蓝牙连接智能手表。
| Network Type 网络类型 | Geographical Range 地理范围 | Typical Use 典型用途 |
|---|---|---|
| LAN | Building or campus 建筑物或校园 | School computer lab 学校机房 |
| WAN | City to global 城市到全球 | The internet 互联网 |
| PAN | A few metres 几米以内 | Bluetooth devices 蓝牙设备 |
3. Network Topologies: Star, Bus, Ring and Mesh | 网络拓扑：星型、总线型、环型和网状
Network topology refers to the physical or logical arrangement of devices in a network. The four common topologies are star, bus, ring and mesh, each with different performance and reliability characteristics.
网络拓扑指网络中设备的物理或逻辑排列方式。四种常见拓扑是星型、总线型、环型和网状，每种拓扑在性能和可靠性方面各有特点。
In a star topology, every device connects to a central switch or hub. If one cable fails, only that device is affected, but the central device is a single point of failure.
在星型拓扑中，每台设备都连接到中央交换机或集线器。如果某条电缆发生故障，只有该设备受影响，但中央设备是单点故障。
A bus topology uses a single backbone cable to which all devices attach. It is cheap to install but a break in the backbone can bring down the entire network segment.
总线型拓扑使用一条主干电缆连接所有设备。它的安装成本较低，但主干一旦断裂，整个网段都可能瘫痪。
A ring topology connects devices in a closed loop, with data travelling in one direction. It can be efficient, but a failure in any node or cable can disrupt the whole loop unless dual rings are used.
环型拓扑将设备连接成一个闭环，数据单向传输。它可能很高效，但任何节点或电缆发生故障都会中断整个环路，除非使用双环结构。
A mesh topology connects many devices to each other, providing multiple paths for data. It is highly reliable and often used in WANs, but it requires more cabling and configuration effort.
网状拓扑将许多设备彼此相连，为数据提供多条路径。它非常可靠，常用于广域网，但需要更多布线和配置工作。
4. Network Hardware: Router, Switch, Hub and Modem | 网络硬件：路由器、交换机、集线器和调制解调器
Network hardware includes the physical devices that connect computers and route data. Common examples are routers, switches, hubs and modems, each serving a different role.
网络硬件包括连接计算机和路由数据的物理设备。常见的例子有路由器、交换机、集线器和调制解调器，它们各自承担不同的角色。
A router forwards data packets between different networks, often connecting a home LAN to the internet. It uses IP addresses to determine the best path for each packet.
路由器在不同网络之间转发数据包，通常将家庭局域网连接到互联网。它使用IP地址为每个数据包确定最佳路径。
A switch operates within a LAN and sends data only to the intended recipient device by learning MAC addresses. This reduces unnecessary traffic compared with older hubs.
交换机在局域网内工作，通过学习MAC地址仅将数据发送到目标设备。与较旧的集线器相比，这减少了不必要的流量。
A hub is a simple device that broadcasts incoming data to all connected devices. It is rarely used today because it creates collisions and wastes bandwidth.
集线器是一种简单设备，将收到的数据广播给所有连接的设备。如今很少使用，因为它会产生冲突并浪费带宽。
A modem converts digital signals from a computer into analogue signals suitable for transmission over telephone or cable lines, and vice versa. Many home devices combine modem and router functions.
调制解调器将计算机的数字信号转换为适合通过电话线或电缆传输的模拟信号，反之亦然。许多家用设备集成了调制解调器和路由器的功能。
5. IP Addresses and MAC Addresses | IP地址和MAC地址
Every device on a network needs an address so that data can reach it. IP addresses and MAC addresses are two important addressing systems used in computer networks.
网络上的每台设备都需要一个地址，以便数据能够到达它。IP地址和MAC地址是计算机网络中使用的两种重要寻址系统。
An IP address is a logical address assigned to a device on a network. IPv4 addresses are written as four decimal numbers separated by dots, for example 192.168.1.10, while IPv6 uses a longer hexadecimal format.
IP地址是分配给网络设备的逻辑地址。IPv4地址写成由点分隔的四个十进制数，例如192.168.1.10，而IPv6则使用更长的十六进制格式。
A MAC address is a physical address permanently assigned to a network interface card by its manufacturer. It is usually written as six pairs of hexadecimal digits, such as 00:1A:2B:3C:4D:5E.
MAC地址是由制造商永久分配给网卡的物理地址。它通常写成六对十六进制数字，例如00:1A:2B:3C:4D:5E。
IP addresses identify a device globally and can change when the device moves to another network, while MAC addresses stay the same and identify the hardware within a local segment.
IP地址在全局范围内标识设备，并可能在设备切换到另一网络时改变；而MAC地址保持不变，用于在本地网段内标识硬件。
6. The Internet and the World Wide Web | 互联网与万维网
The internet is a global network of interconnected networks that uses the TCP/IP protocol suite to transmit data. It provides the infrastructure for services such as email, file transfer and the World Wide Web.
互联网是一个由互连网络组成的全球网络，使用TCP/IP协议族传输数据。它为电子邮件、文件传输和万维网等服务提供基础设施。
The World Wide Web, or WWW, is a collection of web pages and resources accessed through the internet using the HTTP and HTTPS protocols. It is not the same as the internet itself.
万维网（WWW）是通过互联网使用HTTP和HTTPS协议访问的网页和资源的集合。它与互联网本身并不相同。
Web pages are identified by Uniform Resource Locators, or URLs, such as https://www.example.com/index.html. The domain name system, or DNS, translates these names into IP addresses.
网页通过统一资源定位符（URL）标识，例如https://www.example.com/index.html。域名系统（DNS）将这些名称转换为IP地址。
When a user enters a URL, the browser sends an HTTP request to the server, which responds with the requested page. This client-server interaction is the foundation of web browsing.
当用户输入URL时，浏览器向服务器发送HTTP请求，服务器以所请求的页面进行响应。这种客户端-服务器交互是网页浏览的基础。
7. Data Transmission: Wired and Wireless | 数据传输：有线与无线
Data can be transmitted over wired media such as copper cables and fibre optic cables, or wirelessly using radio waves, microwaves or infrared signals.
数据可以通过有线介质（如铜缆和光纤电缆）传输，也可以使用无线电波、微波或红外信号进行无线传输。
Fibre optic cables use light to carry data at very high speeds over long distances with minimal interference. Copper cables such as Ethernet are cheaper but more susceptible to electromagnetic interference.
光纤电缆利用光以极高的速度在长距离上传输数据，并且干扰极小。以太网等铜缆价格更低，但更容易受到电磁干扰。
Wireless technologies include Wi-Fi, which is based on the IEEE 802.11 standards, and Bluetooth, which is designed for short-range personal devices. Wireless networks offer mobility but can suffer from signal interference and security risks.
无线技术包括基于IEEE 802.11标准的Wi-Fi，以及为短距离个人设备设计的蓝牙。无线网络提供了移动性，但可能受到信号干扰并存在安全风险。
Bandwidth, measured in bits per second, and latency, measured in milliseconds, are key factors that affect transmission performance. Higher bandwidth allows more data to be sent per second, while lower latency improves responsiveness.
带宽以比特每秒为单位，延迟以毫秒为单位，是影响传输性能的关键因素。较高的带宽每秒可发送更多数据，而较低的延迟则提高了响应速度。
8. Network Protocols: TCP/IP, HTTP, HTTPS and FTP | 网络协议：TCP/IP、HTTP、HTTPS和FTP
A network protocol is a set of rules that allows devices to communicate reliably. The TCP/IP suite is the foundation of the internet and includes many protocols working at different layers.
网络协议是允许设备可靠通信的一组规则。TCP/IP协议族是互联网的基础，包含在不同层工作的许多协议。
TCP, or Transmission Control Protocol, breaks data into packets, numbers them and reassembles them at the destination. It also checks for errors and requests retransmission of lost packets.
TCP（传输控制协议）将数据拆分为数据包，对其进行编号，并在目的地重新组装。它还会检查错误并请求重传丢失的数据包。
IP, or Internet Protocol, is responsible for addressing and routing packets across networks. It ensures that each packet is directed towards its destination IP address.
IP（互联网协议）负责跨网络对数据包进行寻址和路由。它确保每个数据包被导向其目标IP地址。
HTTP, or Hypertext Transfer Protocol, is used to transfer web pages between servers and clients. HTTPS is the secure version that encrypts data using TLS or SSL to protect it from interception.
HTTP（超文本传输协议）用于在服务器和客户端之间传输网页。HTTPS是安全版本，使用TLS或SSL对数据加密，防止被窃听。
FTP, or File Transfer Protocol, is used to upload and download files between a client and a server. It is commonly used for website maintenance and sharing large files.
FTP（文件传输协议）用于在客户端和服务器之间上传和下载文件。它通常用于网站维护和共享大文件。
9. Network Security: Threats and Protection | 网络安全：威胁与防护
Network security protects data and resources from unauthorised access, misuse and damage. Common threats include malware, phishing, denial-of-service attacks and man-in-the-middle interception.
网络安全保护数据和资源免受未经授权的访问、滥用和破坏。常见威胁包括恶意软件、网络钓鱼、拒绝服务攻击和中间人拦截。
Malware is malicious software such as viruses, worms, Trojans and ransomware. It can corrupt data, steal information or lock users out of their systems.
恶意软件是病毒、蠕虫、木马和勒索软件等恶意程序。它们可能损坏数据、窃取信息或将用户锁在系统之外。
Phishing uses fake emails or websites to trick users into revealing passwords and personal data. Users should check sender addresses and avoid clicking suspicious links.
网络钓鱼使用虚假电子邮件或网站欺骗用户泄露密码和个人数据。用户应检查发件人地址并避免点击可疑链接。
Protection measures include firewalls, which filter incoming and outgoing traffic, and encryption, which scrambles data so that only authorised parties can read it. Strong passwords and regular software updates are also essential.
防护措施包括防火墙（过滤传入和传出的流量）和加密（将数据置乱，只有授权方才能读取）。强密码和定期软件更新也至关重要。
Authentication methods such as two-factor authentication add a second layer of security by requiring a code or biometric confirmation in addition to a password.
双因素认证等身份验证方法通过要求除密码外的验证码或生物识别确认，增加了第二层安全保护。
10. The Client-Server Model and Cloud Computing | 客户端-服务器模型与云计算
The client-server model is a computing architecture in which clients request services and servers provide them. A web browser acting as a client requests pages from a web server, which stores and delivers the content.
客户端-服务器模型是一种计算架构，其中客户端请求服务，服务器提供服务。作为客户端的网络浏览器向存储并传送内容的网络服务器请求页面。
Servers are usually powerful computers that run continuously and can handle many client requests at once. Examples include file servers, email servers and database servers.
服务器通常是功能强大的计算机，可连续运行并能同时处理许多客户端请求。示例包括文件服务器、电子邮件服务器和数据库服务器。
Cloud computing allows users to access computing resources such as storage, software and processing power over the internet instead of using local hardware. Services are often split into Infrastructure as a Service, Platform as a Service and Software as a Service.
云计算允许用户通过互联网访问存储、软件和处理能力等计算资源，而不必使用本地硬件。服务通常分为基础设施即服务、平台即服务和软件即服务。
Cloud storage enables files to be stored on remote servers and synchronised across devices. This improves accessibility and backup, but it depends on a stable internet connection and raises privacy considerations.
云存储让文件存储在远程服务器上并可在不同设备间同步。这提高了可访问性和备份能力，但依赖稳定的互联网连接，并引起隐私方面的考虑。
Modern applications increasingly use a hybrid model, combining local processing on client devices with cloud-based services for storage, machine learning and real-time collaboration.
现代应用越来越多地采用混合模式，将客户端设备上的本地处理与云端的存储、机器学习和实时协作服务结合起来。
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