📚 Network Basics for IGCSE AQA Computer Science | IGCSE AQA 计算机：网络基础 考点精讲
Computer networks form the backbone of modern communication. In this revision guide, we cover the key concepts required for the IGCSE AQA Computer Science syllabus: network types, topologies, hardware, addressing, packet switching, protocols, and security essentials. Understanding these fundamentals will help you answer exam questions confidently.
计算机网络是现代通信的支柱。在本考点精讲中，我们涵盖了IGCSE AQA计算机科学大纲所要求的核心概念：网络类型、拓扑结构、硬件、寻址、数据包交换、协议以及安全基础。理解这些基础知识将帮助你自信地回答考试题目。
1. Types of Networks: LAN and WAN | 网络类型：局域网与广域网
A Local Area Network (LAN) is a network that covers a small geographical area, typically a single building or site. It is usually owned, controlled and managed by a single person or organisation. LANs offer high data transfer rates and low latency.
局域网（LAN）覆盖较小的地理范围，通常是单个建筑物或场所。它通常由单一个人或组织拥有、控制和管理。局域网提供高数据传输速率和低延迟。
A Wide Area Network (WAN) spans a large geographical area, often a country or continent. The internet is the largest WAN. WANs may use leased telecommunication lines or satellite links, and are generally slower and more expensive than LANs.
广域网（WAN）跨越较大的地理范围，通常是一个国家或大洲。互联网是最大的广域网。广域网可能使用租用的电信线路或卫星连接，一般而言比局域网速度慢且成本更高。
The key difference is scale and ownership. LANs are private and fast; WANs connect multiple LANs over long distances and are maintained by multiple organisations or carriers.
关键区别在于规模和所有权。局域网是私有的、速度快；广域网将多个局域网长距离连接起来，并由多个组织或运营商维护。
2. Network Topologies: Star, Bus and Ring | 网络拓扑结构：星型、总线型与环型
In a star topology, all devices are connected to a central switch or hub. If one cable fails, only that device is affected. However, if the central device fails, the whole network goes down. It is easy to add new devices without disrupting the network.
在星型拓扑中，所有设备都连接到中央交换机或集线器。如果一根电缆出现故障，只有该设备受影响。但如果中央设备故障，整个网络就会瘫痪。添加新设备很容易，不会中断网络。
A bus topology uses a single main cable (the backbone) to which all devices connect. It is cheap and simple to install, but if the backbone cable breaks, the entire network fails. Performance degrades as more devices are added.
总线型拓扑使用一根主干电缆（骨干），所有设备都连接到这根电缆上。它便宜且安装简单，但是如果骨干电缆断裂，整个网络就会瘫痪。随着更多设备加入，性能会下降。
A ring topology connects devices in a closed loop, with data travelling in one direction. Each device repeats the signal. Failure of a single node or cable break can bring down the whole network unless a dual ring is used.
环型拓扑将设备连接成一个闭环，数据单向传输。每个设备都转发信号。单个节点故障或电缆断裂都可能使整个网络瘫痪，除非使用双环结构。
Exam tip: Be able to compare star and bus topologies – particularly cost, reliability and scalability.
考试提示：要能够比较星型和总线型拓扑——尤其是成本、可靠性和可扩展性。
3. Network Hardware Essentials | 网络硬件基础
A Network Interface Card (NIC) is required for a device to connect to a network. It provides a unique MAC address and converts data into electrical or optical signals.
网卡（NIC）是设备连接网络所必需的。它提供唯一的MAC地址，并将数据转换为电信号或光信号。
A switch connects devices within a LAN and uses MAC addresses to forward data only to the intended recipient. This reduces unnecessary traffic compared to a hub.
交换机在局域网内连接设备，并使用MAC地址仅将数据转发给目标接收方。与集线器相比，这减少了不必要的流量。
A router connects different networks, such as a home LAN to the internet. It forwards data packets using IP addresses and can act as a gateway.
路由器连接不同的网络，例如家庭局域网与互联网。它使用IP地址转发数据包，并可作为网关使用。
A wireless access point (WAP) allows wireless devices to connect to a wired network using Wi-Fi, making mobility possible.
无线接入点（WAP）允许无线设备通过Wi-Fi连接到有线网络，实现移动性。
4. IP Addressing and MAC Addresses | IP地址与MAC地址
An IP address is a logical address used to identify a device on a network. IPv4 addresses are 32 bits long, e.g., 192.168.1.5. IPv6 uses 128 bits to provide a vastly larger address space. IP addresses can be static or dynamically assigned by DHCP.
IP地址是用于标识网络中设备的逻辑地址。IPv4地址长度为32位，例如192.168.1.5。IPv6使用128位，提供了极大的地址空间。IP地址可以是静态分配的，也可以通过DHCP动态分配。
A MAC address (Media Access Control) is a 48-bit physical address burned into the NIC by the manufacturer. It is unique to each network interface and usually stays with the hardware. MAC addresses are used within a LAN for switching.
MAC地址（媒体访问控制）是由制造商烧录进网卡的48位物理地址。每个网络接口的MAC地址是唯一的，通常跟随硬件。MAC地址在局域网内用于交换。
Know the difference: IP addresses work at the Internet layer (Layer 3) and can change; MAC addresses work at the Network Access layer (Layer 2) and are permanent identifiers.
要了解区别：IP地址工作于网际层（第三层）可以改变；MAC地址工作于网络接入层（第二层），是永久的标识符。
5. Packet Switching – How Data Travels | 数据包交换——数据如何传输
In packet switching, a message is broken into small chunks called packets. Each packet contains a header with source and destination IP addresses, a sequence number and error-checking information.
在数据包交换中，消息被分割成小的数据块，称为数据包。每个数据包包含一个包头，其中有源IP地址和目标IP地址、序列号以及差错校验信息。
Packets can take different routes across the network. Routers examine the destination IP address and forward each packet independently. At the destination, the packets are reassembled into the original message using the sequence numbers.
数据包可以通过网络中的不同路径传输。路由器检查目标IP地址并独立转发每个数据包。在接收端，数据包根据序列号重新组装成原始消息。
Packet switching is efficient because network links are shared; if a part of the network fails, packets can be rerouted. This contrasts with circuit switching, which establishes a dedicated path for the duration of a call.
数据包交换效率高，因为网络链路是共享的；如果网络某一部分出现故障，数据包可以重新路由。这与电路交换形成对比，电路交换在通话期间建立专用路径。
6. Protocols and TCP/IP Layered Model | 协议与TCP/IP分层模型
A protocol is a set of rules that governs how data is transmitted over a network. The TCP/IP model organises these protocols into four layers: Application, Transport, Internet, and Network Access.
协议是管理数据如何通过网络传输的一组规则。TCP/IP模型将这些协议组织为四个层次：应用层、传输层、网际层和网络接入层。
- Application Layer: provides services directly to user applications – e.g., HTTP, FTP, SMTP.
- Transport Layer: ensures reliable data transfer (TCP) or fast, connectionless communication (UDP).
- Internet Layer: handles routing and addressing using IP.
- Network Access Layer: deals with physical transmission and MAC addresses.
- 应用层：直接为用户应用程序提供服务，如HTTP、FTP、SMTP。
- 传输层：确保可靠数据传输（TCP）或快速、无连接通信（UDP）。
- 网际层：使用IP处理路由和寻址。
- 网络接入层：处理物理传输和MAC地址。
Because the model is layered, each layer only needs to communicate with the layer directly above or below, simplifying protocol design and allowing devices from different manufacturers to work together.
由于模型是分层的，每一层只需与直接的上层或下层通信，这简化了协议设计，并使不同厂商的设备能够协同工作。
7. Common Application Layer Protocols | 常见应用层协议
HTTP (Hypertext Transfer Protocol) is used to request and deliver web pages. It works in a client-server fashion: a web browser (client) requests a page, and a web server sends the HTML content back. HTTPS is the secure, encrypted version using SSL/TLS.
HTTP（超文本传输协议）用于请求和传递网页。它以客户端-服务器的方式工作：网页浏览器（客户端）请求页面，网页服务器将HTML内容发回。HTTPS是使用SSL/TLS加密的安全版本。
FTP (File Transfer Protocol) enables the transfer of files between computers. It is often used to upload webpages to a server or download large files.
FTP（文件传输协议）实现在计算机之间传输文件。它常被用来将网页上传到服务器或下载大型文件。
SMTP (Simple Mail Transfer Protocol) is used for sending
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