Intro to Network
局域网中的物理地址通信
局域网（LAN）中，节点通过物理MAC地址进行通信，交换机能快速学习连接端口的MAC地址以实现有效传输。 由于MAC地址全球唯一且无序，且无法定位其地理位置，MAC地址不适合跨网络通信。
网络层与IP地址的必要性
物理地址学习机制（如ARP协议）仅适用于单一网络段，无法扩展到更大范围。 因此，需要网络层和互联网协议（IP）及其IP地址来实现跨网络的通信。
IP数据报及其封装
IP数据报被封装在以太网帧的有效载荷中进行传输。 课程将详细介绍IP数据报头的各个字段及其功能，帮助理解网络层数据传输的细节。
IP地址的结构与表示
IP地址是由32位二进制数构成，分为四个八位字节（octet），每个八位字节用十进制表示，范围是0到255。 这种表示方法称为点分十进制表示法，例如12.34.56.78是有效的IP地址，而123.456.789.100不是。
IP地址的分配与层级
IP地址不是由硬件厂商决定，而是分配给不同组织和公司的大块地址空间，形成层级结构。 例如，IBM拥有所有以9开头的IP地址，这使得路由器只需知道如何将数据包送到IBM的路由器即可。
动态与静态IP地址
IP地址属于网络，而非设备，设备在不同网络中会获得不同的IP地址。 动态主机配置协议（DHCP）可以自动为设备分配动态IP地址，通常用于客户端。 静态IP地址需要手动配置，通常用于服务器和网络设备。
IP数据报的结构
IP数据报由两个主要部分组成：头部（header）和负载（payload）。 头部包含多个字段，如版本号、头部长度、服务类型、总长度、标识、标志、分片偏移、存活时间（TTL）、协议、头部校验和、源IP地址和目的IP地址等。
关键字段及其功能
版本字段指明使用的IP协议版本，常见的是IPv4和IPv6。 标识字段用于标记分片的数据包，帮助接收端重组。 标志和分片偏移字段用于控制和指示数据包的分片情况。 TTL字段防止数据包在网络中无限循环，通过每经过一个路由器减一，直到为零时丢弃数据包。 协议字段指示传输层协议，如TCP或UDP。
IP数据报与以太网帧的关系
IP数据报作为以太网帧的负载被封装传输。 IP数据报的负载部分通常是传输层的TCP或UDP数据包。 这种分层封装体现了网络通信中层次化设计的原则。
ARP的作用与工作流程
当设备需要发送数据到某个IP地址时，必须知道对应的MAC地址以构建以太网帧。 如果本地ARP表中没有目标IP的MAC地址，设备会发送ARP广播请求，询问网络中对应IP的MAC地址。
ARP响应与缓存机制
拥有该IP地址的设备收到ARP请求后，会发送ARP响应，告知其MAC地址。 发送设备收到响应后，更新本地ARP表，避免下次重复广播请求。
ARP表的维护
ARP表存储IP地址与MAC地址的对应关系。 表项通常会在一段时间后过期，以适应网络环境的变化。
子网划分的定义与目的
子网划分是将一个大型网络拆分成多个较小的子网络（子网）的过程。 这样做是为了避免单个大型网络中设备过多，导致路由器负担过重。
IP地址分类与路由
IP地址通过地址类（如Class A）划分，帮助核心路由器识别网络部分。 核心路由器将数据包发送到对应网络的网关路由器，后者负责将数据传递给具体主机。
子网划分的实际应用
一个Class A网络包含超过1600万个IP地址，单一网关路由器难以管理。 通过子网划分，每个子网拥有自己的网关路由器，提升网络管理的灵活性和效率。
子网ID的引入
IP地址由网络ID和主机ID组成，子网划分时，部分主机ID位被用作子网ID。 子网ID使得单个32位IP地址能在多个网络中准确传递。
子网掩码的作用与表示
子网掩码是一个32位的二进制数，前面一段是连续的1，后面是0，用来区分子网ID和主机ID。 例如，255.255.255.0对应24个1和8个0，表示前24位是子网ID，后8位是主机ID。
子网大小与地址分配
子网大小由子网掩码决定，主机ID位数决定可用主机数量。 通常主机地址数为2的主机ID位数次方减2（减去网络地址和广播地址），如255.255.255.0子网掩码下，主机地址范围是1到254。 子网掩码也可以用斜线表示法，如/27表示27个1后跟5个0。
二进制的数学运算与逻辑运算
二进制加法规则简单，只有四种情况，1+1进位为10。 逻辑运算符AND和OR用于计算机逻辑判断，AND要求两个输入都为1才输出1，OR只要有一个输入为1就输出1。
二进制在网络中的应用：子网掩码
子网掩码通过AND运算确定IP地址的网络部分，帮助计算机判断两个IP是否在同一网络。 通过将IP地址和子网掩码进行二进制AND操作，得到网络ID，从而实现网络划分和管理。
IP地址分类与子网划分的局限性
传统IP地址分为A、B、C类，网络ID长度固定，导致网络数量和大小不灵活。 许多企业需要的主机数量介于类C和类B之间，导致使用多个相邻类C网络，增加路由表条目。
CIDR的引入与工作原理
CIDR放弃了传统的地址分类，使用子网掩码灵活划分网络，网络ID和子网ID合并。 采用斜杠表示法（如9.100.100.100/24）简化网络标识，支持任意大小的网络。
CIDR的优势
减少路由表条目数量，提高路由效率。 支持连续地址块合并，满足不同规模企业的需求。 提供更多可用主机地址，减少地址浪费。
路由的基本概念
路由是连接多个网络并允许数据包在它们之间传输的过程。 路由器是具有至少两个网络接口的设备，根据数据包的目标地址决定转发路径。
路由的工作流程
路由器接收数据包，检查目标IP地址。 路由器查找路由表，确定最佳路径并将数据包转发到相应接口。 数据包经过多个路由器，逐步接近最终目的地。
路由的实际示例与复杂性
简单示例中，路由器连接两个网络，转发数据包到目标网络。 复杂示例中，数据包可能经过多个路由器和网络，路由器通过路由表选择最短路径。 互联网中的路由器通常连接多个网络，形成网状结构以保证路径冗余和可靠性。
路由协议的分类
路由协议帮助路由器相互通信，分享网络路径信息，分为内部网关协议和外部网关协议。 内部网关协议用于单一自治系统内的路由信息交换，自治系统指由单一网络运营商控制的网络集合。
距离矢量协议
距离矢量协议通过发送路由表中网络的跳数信息给邻居路由器，更新路径信息。 这种协议只了解直接邻居的状态，反应网络变化较慢，适合简单网络环境。
链路状态协议
链路状态协议通过广播每个接口的链路状态信息，使所有路由器了解整个自治系统的网络拓扑。 每个路由器基于完整的网络信息运行算法计算最佳路径，需更多内存和计算资源，适合复杂网络环境。
外部网关协议的作用
用于不同自治系统边缘路由器之间的数据通信，连接不同组织的网络。 是互联网正常运行的关键，帮助核心路由器了解自治系统以正确转发流量。
自治系统与核心路由器
互联网由大量自治系统组成，核心路由器的主要目标是将数据传送到自治系统的边缘路由器。 自治系统是已知且定义好的网络集合，核心路由器通过自治系统编号识别它们。
自治系统编号（ASN）和IANA
IANA（互联网号码分配局）负责分配IP地址和自治系统编号，确保互联网地址的唯一性和规范管理。 ASN是32位数字，通常以单一十进制数表示，用于标识自治系统，区别于分段表示的IP地址。
We’ve covered a few different routing protocol types, but we haven’t discussed the details of how the actual implementation of these protocols might matter.
Many network protocols are implemented based on specifications published by the Internet Engineering Task Force (IETF) . We'll cover this in more detail in a future lesson!
The most common distance vector protocols are RIP, or Routing Information Protocol ( IETF RFC2453 ), and EIGRP, or Enhanced Interior Gateway Routing Protocol ( Cisco documentation ). The most common link state protocol is OSPF, or Open Shortest Path First ( IETF RFC2328 ).
In terms of exterior gateway protocols, there is only one in use today. The entire Internet needs to agree on how to exchange this sort of information, so a single standard has emerged. This standard is known as BGP, or Border Gateway Protocol ( IETF RFC4271
New terms and their definitions: Course 2 Module 6 新术语及其定义：第 2 门课程的第 6 个模块 Cloud computing: The concept and technological approach of accessing data, using applications, storing files, etc. from anywhere in the world as long as you have an internet connection
Error detection: The ability for a protocol or program to determine that something went wrong
Error recovery: The ability for a protocol or program to attempt to fix an error
Flow label field: 20-bit field that's used in conjunction with the traffic class field for routers to make decisions about the quality of service level for a specific datagram
Hop limit field: An 8-bit field that's identical in purpose to the TTL field in an IPv4 header
Host file: A simple text file that consists of one or more lines, each of which contains a hostname and an IP address 主机文件：这是一种简单的文本文件，由多行组成。每行包含一个主机名和一个 IP 地址。
Hybrid cloud: A cloud computing environment that combines on-premises private cloud resources with public cloud resources 混合云：一种将本地私有云资源与公有云资源相结合的云计算环境。
Hypervisor: A piece of software that runs and manages virtual machines while also 虚拟机管理程序：一种用于运行和管理虚拟机的软件，同时还能……
offering guests a virtual operating platform that's indistinguishable from actual hardware 为客人提供一种与真实硬件毫无区别的虚拟操作平台。
ICMP: Internet control message protocol is used by router or remote hosts to communicate error messages when network problems prevent delivery of IP packets ICMP：互联网控制消息协议。当网络问题导致 IP 数据包无法传输时，路由器或远程主机会利用该协议来发送错误消息。
ICMP payload: Piece of the packet which lets the recipient of the message know which of their transmissions caused the error being reported ICMP 有效载荷：这是数据包中的一部分，它能让消息的接收方知道，是哪次传输导致了所报告的错误。
Infrastructure as a Service (IaaS): A subset of cloud computing where a network and servers are provided for customers to run their services 基础设施即服务（IaaS）：云计算的一个子领域，它为用户提供网络和服务器资源，以便用户能够在其上运行各种应用程序和服务。
IPv6 tunnel brokers: Companies that provide IPv6 tunneling endpoints for you, so you don't have to introduce additional equipment to your network IPv6 隧道中介服务：这些公司会为你提供 IPv6 隧道连接功能，这样一来，你就不必在网络中额外添加任何设备了。
IPv6 tunnel: IPv6 tunnel servers on either end of a connection take incoming IPv6 traffic and encapsulate it within traditional IPv4 datagrams IPv6 隧道：在连接的两端，IPv6 隧道服务器会接收进入的 IPv6 数据包，并将其封装在传统的 IPv4 数据报中。
Link-local unicast address: Allow for local network segment communications and are configured based upon a host's MAC address 链路本地单播地址：用于实现局域网内的通信，其配置依据是主机的 MAC 地址。
Loopback address: An IP address that always points to itself. This type of address is used to test internal pathing through the TCP/IP protocols 回环地址：一种始终指向自身的 IP 地址。这种地址被用于通过 TCP/IP 协议来测试网络内部的路径传输情况。
Multicast: A way of addressing groups of hosts all at once 多播：一种能够同时向多个主机发送数据的方式
Next header field: Defines what kind of header is immediately after this current one 下一个标题字段：用于指定紧随当前标题之后的标题类型。
Payload length field: 16-bit field that defines how long the data payload section of the datagram is 有效载荷长度字段：这是一个 16 位的字段，用于指定数据报中有效载荷部分的长度。
Platform as a service: A subset of cloud computing where a platform is provided for customers to run their services 平台即服务：云计算的一种形式，指的是向用户提供用于运行各类服务的平台。
Private cloud: When a company owns the services and the rest of the cloud infrastructure, whether on-site or in a remote data center 私有云：指企业拥有所有的云服务及相关基础设施，这些设施可以位于企业内部，也可以位于远程的数据中心中。
Public cloud: The cloud services provided by a third party 公有云：由第三方提供的云服务
Public DNS servers: Name servers specifically set up so that anyone can use them for free 公共 DNS 服务器：专门为所有人免费使用而设置的域名解析服务器。
Registrar: An organization responsible for assigning individual domain names to other organizations or individuals 注册机构：负责将各个域名分配给其他组织或个人的机构。
Software as a Service (SaaS): A way of licensing the use of software to others while keeping that software centrally hosted and managed 软件即服务（SaaS）：这是一种将软件的使用许可授权给其他用户的模式，同时，该软件仍由第三方进行集中托管和管理。
Traffic class field: An 8-bit field that defines the type of traffic contained within the IP datagram and allows for different classes of traffic to receive different priorities 流量类别字段：这是一个 8 位字段，用于指定 IP 数据包中所包含的流量类型。这样一来，不同类型的流量就可以被赋予不同的优先级。
Version field: First field in an IP header that specifies the version of IP 版本字段：IP 头部中的第一个字段，用于指定 IP 的版本。
Virtualization: A single physical machine called a host runs many individual virtual instances called guests 虚拟化技术：一台被称为“主机”的物理机器上，可以运行多个被称为“虚拟机”的独立虚拟系统。
Terms and their definitions from previous modules 前几个模块中出现的术语及其定义 A
A record: The most common resource record, used to point a certain domain name at a certain IPv4 IP address 一种记录类型：最常见的资源记录，用于将某个域名映射到特定的 IPv4 地址上。
ACK flag: One of the TCP control flags. ACK is short for acknowledge. A value of one in this field means that the acknowledgment number field should be examined ACK 标志：这是 TCP 控制标志之一。ACK 是“确认”的缩写。该字段的值为 1 时，意味着需要查看确认号字段。
Acknowledgement number: The number of the next expected segment in a TCP sequence 确认号：TCP 序列中下一个预期数据段的编号
Ad-Hoc network: A network configuration without supporting network infrastructure. Every device involved with the ad-hoc network communicates with every other device within range, and all nodes help pass along messages 临时网络：一种不依赖任何固定网络基础设施的网络架构。参与该网络的每个设备都会与范围内其他所有设备进行通信，所有节点共同协作来传递信息。
Address class system: A system which defines how the global IP address space is split up 地址分类系统：一种用于规定全球 IP 地址空间如何被划分的机制
Address Resolution Protocol (ARP): A protocol used to discover the hardware address of a node with a certain IP address 地址解析协议（ARP）：一种用于确定具有特定 IP 地址的节点的硬件地址的协议。
Anycast: A technique that's used to route traffic to different destinations depending on factors like location, congestion, or link health 任播：这是一种根据位置、网络拥堵情况或链路状态等因素，将数据流量路由到不同目的地的技术。
Application layer payload: The entire contents of whatever data applications want to send to each other 应用层有效载荷：各种数据应用想要相互发送的所有数据内容。
Application layer: The layer that allows network applications to communicate in a way they understand 应用层：这一层使得网络应用程序能够以它们能够理解的方式相互通信。
ARP table: A list of IP addresses and the MAC addresses associated with them ARP 表：包含 IP 地址及其对应的 MAC 地址的列表
ASN: Autonomous System Number is a number assigned to an individual autonomous system ASN：自治系统编号是指分配给各个独立自治系统的编号。
Asymmetric Digital Subscriber Line (ADSL): A device that establishes data connections across phone lines and different speeds for uploading and downloading data 非对称数字用户线（ADSL）：一种通过电话线来建立数据连接的技术，支持不同的上传和下载速率。
Automatic allocation: A range of IP addresses is set aside for assignment purposes 自动分配：会预先分配一定范围的 IP 地址以供使用。
B
Baud rate: A measurement of how many bits could be passed across a phone line in a second 波特率：衡量每秒可以通过电话线传输的比特数的指标。
Bit: The smallest representation of data that a computer can understand 比特：计算机能够理解的最小的数据单位。
Bluetooth: The most common short range wireless network 蓝牙：最常用的短距离无线网络技术
Border Gateway Protocol (BGP): A protocol by which routers share data with each other 边界网关协议（BGP）：一种让路由器之间能够互相交换数据的协议
Broadband: Any connectivity technology that isn't dial-up Internet 宽带：任何不属于拨号上网方式的连接技术
Broadcast address: A special destination used by an Ethernet broadcast composed by all Fs 广播地址：一种特殊的地址，用于以太网广播，该广播信号由所有的“1”比特组成。
Broadcast: A type of Ethernet transmission, sent to every single device on a LAN 广播：这是一种以太网传输方式，会将数据发送给局域网中的每一台设备。
C
Cable categories: Groups of cables that are made with the same material. Most network cables used today can be split into two categories, copper and fiber 电缆类别：由相同材料制成的电缆组合。目前使用的大多数网络电缆可分为两类：铜缆和光纤缆。
Cable modem termination system: Connects lots of different cable connections to an ISP's core network 电缆调制解调器终端系统：将各种不同的电缆连接方式与互联网服务提供商的核心网络相连接。
Cable modem: A device that sits at the edge of a consumer's network and connects it to the cable modem termination system 电缆调制解调器：这是一种位于用户网络终端的设备，用于将用户网络与电缆调制解调器终端系统相连。
Cables: Insulated wires that connect different devices to each other allowing data to be transmitted over them 电缆：一种具有绝缘层的导线，用于连接各种设备，从而实现数据的传输。
Caching and recursive name servers: They are generally provided by an ISP or your local network, and their purpose is to store domain name lookups for a certain amount of time 缓存和递归名称服务器：这些服务通常由互联网服务提供商或本地网络提供。其作用是暂时存储域名查询结果，以减少重复查询的必要。
Carrier-Sense Multiple Access with Collision Detection (CSMA/CD): CSMA/CD is used to determine when the communications channels are clear and when the device is free to transmit data 载波侦听多路访问/冲突检测机制（CSMA/CD）：CSMA/CD 用于判断通信频道是否处于空闲状态，从而确定设备何时可以开始发送数据。
Channels: Individual, smaller sections of the overall frequency band used by a wireless network 频道：无线网络所使用的整个频段中被划分出来的、较小的独立部分。
Client: A device that receives data from a server 客户端：从服务器接收数据的设备
CLOSE_WAIT: A connection state that indicates that the connection has been closed at the TCP layer, but that the application that opened the socket hasn't released its hold on the socket yet CLOSE_WAIT：这是一种连接状态，表示该连接在 TCP 层面上已经被关闭，但创建该套接字的应用程序尚未释放对该套接字的占用。
CLOSE: A connection state that indicates that the connection has been fully terminated, and that no further communication is possible 已关闭：这是一种连接状态，表示该连接已经完全终止，无法再进行任何通信了。
CNAME: A resource record used to map one domain to another CNAME：一种用于将一个域名映射到另一个域名的资源记录
Collision domain: A network segment where only one device can communicate at a time 冲突域：指在某个网络段中，同一时间只能有一台设备进行通信。
Collision domain: A network segment where only one device can communicate at a time 冲突域：指在某个网络段中，同一时间只能有一台设备进行通信。
Computer networking: The full scope of how computers communicate with each other 计算机网络：计算机之间相互通信的完整机制/过程
Connection-oriented protocol: A data-transmission protocol that establishes a connection at the transport layer, and uses this to ensure that all data has been properly transmitted 面向连接的协议：这是一种在传输层建立连接的数据传输协议。通过这种方式，可以确保所有数据都能被正确地传输。
Connectionless protocol: A data-transmission protocol that allows data to be exchanged without an established connection at the transport layer. The most common of these is known as UDP, or User Datagram Protocol 无连接协议：这是一种数据传输协议，允许在传输层进行数据交换，而无需事先建立连接。其中最著名的无连接协议就是 UDP，即用户数据报协议。
Copper cable categories: These categories have different physical characteristics like the number of twists in the pair of copper wires. These are defined as names like category (or cat) 5, 5e, or 6, and how quickly data can be sent across them and how resistant they are to outside interference are all related to the way the twisted pairs inside are arranged 铜缆的类别：这些类别具有不同的物理特性，比如每对铜线的绞合次数。它们被命名为“5 类”、“5e 类”或“6 类”等。数据在其中的传输速度以及抗外部干扰的能力，都与铜线内部的绞合方式有关。
Crosstalk: Crosstalk is when an electrical pulse on one wire is accidentally detected on another wire 串扰：串扰是指一根导线上的电信号意外地被另一根导线检测到的一种现象。
Cyclical Redundancy Check (CRC): A mathematical transformation that uses polynomial division to create a number that represents a larger set of data. It is an important concept for data integrity and is used all over computing, not just network transmissions 循环冗余校验（CRC）：这是一种通过多项式除法来生成数值的数学算法，该数值能够代表一组更庞大的数据。它是确保数据完整性的重要机制，被广泛应用于各种计算场景中，而不仅仅是网络传输领域。
D
Data offset field: The number of the next expected segment in a TCP packet/datagram 数据偏移量字段：TCP 数据包/数据报中下一个预期数据段的编号
Data packet: An all-encompassing term that represents any single set of binary data being sent across a network link 数据包：这是一个广义的术语，指的是通过网络链路传输的任何一组二进制数据。
Data payload section: Has all of the data of the protocols further up the stack of a frame 数据载荷部分：包含了该帧中各层协议所传输的所有数据。
Datalink layer: The layer in which the first protocols are introduced. This layer is responsible for defining a common way of interpreting signals, so network devices can communicate 数据链路层：这是最早引入各种协议的层次。该层负责定义一种统一的信号解析方式，从而使网络设备能够相互通信。
Demarcate: To set the boundaries of something 划定边界：确定某事物的范围或界限。
Demarcation point: Where one network or system ends and another one begins 分界点：一个网络或系统结束的地方，另一个网络或系统开始的地方。
Demultiplexing: Taking traffic that's all aimed at the same node and delivering it to the proper receiving service 解复用：将所有发往同一个节点的数据流进行分离，然后将其分别传送给相应的接收端。
Destination MAC address: The hardware address of the intended recipient that immediately follows the start frame delimiter 目标 MAC 地址：即接收方的硬件地址，位于帧起始标记之后。
Destination network: The column in a routing table that contains a row for each network that the router knows about 目标网络：路由表中的这一列，其中包含了路由器所知晓的每一个网络的对应条目。
Destination port: The port of the service the TCP packet is intended for 目标端口：TCP 数据包所要发送到的服务所对应的端口
DHCP discovery: The process by which a client configured to use DHCP attempts to get network configuration information DHCP 发现：即配置了 DHCP 功能的客户端试图获取网络配置信息的过程。
DHCP: A technology that assigns an IP address automatically to a new device. It is an application layer protocol that automates the configuration process of hosts on a network DHCP：这是一种能够自动为新设备分配 IP 地址的技术。它属于应用层协议，能够自动完成网络中各主机的配置过程。
Dial-up: Uses POTS for data transfer, and gets its name because the connection is established by actually dialing a phone number 拨号连接：利用普通电话线来传输数据。之所以被称为“拨号连接”，是因为建立连接时需要实际拨打一个电话号码。
DNS zones: A portion of space in the Domain Name System (DNS) that is controlled by an authoritative name server DNS 区域：在域名系统（DNS）中，由权威名称服务器所控制的那部分区域。
Domain Name System (DNS): A global and highly distributed network service that resolves strings of letters, such as a website name, into an IP address 域名系统（DNS）：这是一种全球性且高度分布式的网络服务，它能够将诸如网站名称之类的字符串转换为 IP 地址。
Domain name: A website name; the part of the URL following www. 域名：即网站的名称；是 URL 中“www.”之后的那一部分。
Domain: Used to demarcate where control moves from a top-level domain name server to an authoritative name server 域名：用于界定控制权从顶级域名服务器转移到权威域名服务器的边界。
Dotted decimal notation: A format of using dots to separate numbers in a string, such as in an IP address 点分十进制表示法：一种用点来分隔数字的表示方式，比如在 IP 地址中就采用了这种格式。
DSL: Digital subscriber line was able to send much more data across the wire than traditional dial-up technologies by operating at a frequency range that didn't interfere with normal phone calls DSL：数字用户线路之所以能够比传统的拨号技术传输更多的数据，是因为它工作在不会干扰正常电话通话的频率范围内。
DSLAM: Digital Subscriber Line Access Multiplexers are devices that connect multiple DSL connections to a high-speed digital communications channel DSLAM：数字用户线接入多路复用器是一种能够将多条 DSL 线路连接到高速数字通信通道上的设备。
Duplex communication: A form of communication where information can flow in both directions across a cable 双向通信：一种信息可以通过电缆在两个方向上双向传输的通信方式。
Duration field: Specifies how long the total frame is 持续时间字段：用于指定整个帧的时长。
Dynamic allocation: A range of IP addresses is set aside for client devices and one of these IPs is issued to these devices when they request one 动态分配：会预先为客户端设备分配一定范围的 IP 地址。当客户端设备请求获取 IP 地址时，系统会从中选择一个地址分配给该设备。
Dynamic IP address: An IP address assigned automatically to a new device through a technology known as Dynamic Host Configuration Protocol 动态 IP 地址：通过一种名为“动态主机配置协议”的技术，自动分配给新设备的 IP 地址。
E
ESTABLISHED: Status indicating that the TCP connection is in working order, and both sides are free to send each other data 已建立：该状态表明 TCP 连接正常，双方都可以互相发送数据。
Ethernet frame: A highly structured collection of information presented in a specific order 以太网帧：以特定顺序排列的有条理的信息集合
Ethernet: The protocol most widely used to send data across individual links 以太网：在各个网络连接中，最常被用于数据传输的协议
EtherType field: It follows the Source MAC Address in a dataframe. It's 16 bits long and used to describe the protocol of the contents of the frame EtherType 字段：在数据帧中，它位于源 MAC 地址之后。该字段长度为 16 位，用于说明该数据帧所传输的数据所采用的协议类型。
Exterior gateway: Protocols that are used for the exchange of information between independent autonomous systems 外部网关：用于在各个独立的自治系统之间交换信息的协议
F
Fiber cable: Fiber optic cables contain individual optical fibers which are tiny tubes made of glass about the width of a human hair. Unlike copper, which uses electrical voltages, fiber cables use pulses of light to represent the ones and zeros of the underlying data 光纤电缆：光纤电缆由多根单独的光纤组成。这些光纤其实是极其细小的玻璃管，其直径大约与人类的头发相当。与使用电压传输信号的铜缆不同，光纤电缆利用光脉冲来表示数据的“1”和“0”。
FIN_WAIT: A TCP socket state indicating that a FIN has been sent, but the corresponding ACK from the other end hasn't been received yet FIN_WAIT：一种 TCP 套接字状态，表示已经发送了 FIN 信号，但尚未收到对方的 ACK 响应。
FIN: One of the TCP control flags. FIN is short for finish. When this flag is set to one, it means the transmitting computer doesn't have any more data to send and the connection can be closed FIN：TCP 控制位之一。FIN 是“完成”的缩写。当该位被设置为 1 时，表示发送方计算机没有更多数据需要发送了，此时可以关闭连接。
Firewall: It is a device that blocks or allows traffic based on established rules 防火墙：这是一种根据既定规则来阻止或允许数据流通过的装置。
Five layer model: A model used to explain how network devices communicate. This model has five layers that stack on top of each other: Physical, Data Link, Network, Transport, and Application 五层模型：这是一种用于解释网络设备之间如何进行通信的模型。该模型由五层结构组成，这些层次依次叠加在一起：物理层、数据链路层、网络层、传输层和应用层。
Fixed allocation: Requires a manually specified list of MAC address and the corresponding IPs 固定分配：需要手动指定 MAC 地址及其对应的 IP 地址列表。
Flag field: It is used to indicate if a datagram is allowed to be fragmented, or to indicate that the datagram has already been fragmented 标志字段：该字段用于指示数据报是否可以被分割，或者用于表明该数据报已经被分割过了。
Fragmentation offset field: It contains values used by the receiving end to take all the parts of a fragmented packet and put them back together in the correct order 分片偏移量字段：该字段包含接收端用来将分片后的数据包各部分重新组合起来、并按正确顺序排列所需的数值。
Fragmentation: The process of taking a single IP datagram and splitting it up into several smaller datagrams 分片：将一个 IP 数据报拆分成多个较小的数据报的过程。
Frame check sequence: It is a 4-byte or 32-bit number that represents a checksum value for the entire frame 帧校验序列：这是一个 4 字节或 32 位的数字，用于表示整个帧的校验和值。
Frame check sequence: It is a 4-byte or 32-bit number that represents a checksum value for the entire frame 帧校验序列：这是一个 4 字节或 32 位的数字，用于表示整个帧的校验和值。
Frame control field: 16 bits long, it contains a number of sub-fields that are used to describe how the frame itself should be processed 帧控制字段：长度为 16 位，包含多个子字段，这些子字段用于描述该帧应如何被处理。
Frequency band: A certain section of the radio spectrum that's been agreed upon to be used for certain communications 频段：指在无线电频谱中，经各方同意后专门用于特定通信目的的那部分频谱。
FTP: An older method used for transferring files from one computer to another, but you still see it in use today FTP：这是一种较老的文件传输方式，用于将文件从一台计算机传输到另一台计算机。不过，至今仍有人在使用这种技术。
FTTB: Fiber to the building, fiber to the business or even fiber to the basement, since this is generally where cables to buildings physically enter. FTTB is a setup where fiber technologies are used for data delivery to an individual building FTTB：即光纤到楼宇、光纤到企业，甚至光纤到地下室——因为通常情况下，连接楼宇的电缆都是从这些地方进入楼宇内部的。FTTB 是一种利用光纤技术来向各个楼宇传输数据的解决方案。
FTTH: Fiber to the home. This is used in instances where fiber is actually run to each individual residents in a neighborhood or apartment building FTTH：光纤到户。这种技术适用于将光纤直接铺设到某个社区或公寓楼中每个居民的家中。
FTTN: Fiber to the neighborhood. This means that fiber technologies are used to deliver data to a single physical cabinet that serves a certain amount of the population FTTN：光纤到社区。这意味着利用光纤技术将数据传输到某个特定的物理设备中，该设备再为该区域内的用户提供服务。
FTTP: Fiber to the premises. FTTH and FTTB may both also be referred to as FTTP FTTP：光纤到户。FTTH 和 FTTB 其实都可以被看作是 FTTP 的一种形式。
FTTX: Stands for fiber to the X, where the X can be one of many things FTTX：指的是“光纤到 X”，这里的“X”可以代表多种不同的含义。
Full duplex: The capacity of devices on either side of a networking link to communicate with each other at the exact same time 全双工：网络连接中，两侧的设备能够同时进行相互通信。
Fully qualified domain name: When you combine all the parts of a domain together 完全限定的域名：将域名的所有组成部分组合在一起时所得到的名称
H
Half-duplex: It means that, while communication is possible in each direction, only one device can be communicating at a time 半双工：这意味着，虽然两个方向上的通信都是可行的，但任何时候都只能有一个设备进行通信。
Handshake: A way for two devices to ensure that they're speaking the same protocol and will be able to understand each other 握手协议：这是一种让两台设备确认它们使用的是相同的协议、从而能够互相理解彼此信号的方式。
HDSL: High Bit-rate Digital Subscriber Lines. These are DSL technologies that provision speeds above 1.544 megabits per second HDSL：高比特率数字用户线路。这是一种 DSL 技术，能够提供超过 1.544 兆比特每秒的传输速度。
Header checksum field: A checksum of the contents of the entire IP datagram header 头部校验和字段：对整个 IP 数据报头内容的校验和值
Header length field: A four bit field that declares how long the entire header is. It is almost always 20 bytes in length when dealing with IPv4 标头长度字段：这是一个 4 位的字段，用于说明整个标头的长度。在处理 IPv4 协议时，该字段的长度几乎总是 20 字节。
Hexadecimal: A way to represent numbers using a numerical base of 16 十六进制：一种以 16 为基数来表示数字的方法
Hub: It is a physical layer device that broadcasts data to everything computer connected to it 集线器：这是一种位于物理层的设备，它将数据发送给所有连接到它的计算机。
I
IANA: The Internet Assigned Numbers Authority, is a non-profit organization that helps manage things like IP address allocation IANA：互联网编号分配机构，是一家非营利性组织，负责管理 IP 地址的分配等事务。
Identification field: It is a 16-bit number that's used to group messages together 标识字段：这是一个 16 位的数字，用于将各种消息进行分类/归组。
Instantiation: The actual implementation of something defined elsewhere 实例化：将其他地方定义的内容具体实现出来。
Interface: For a router, the port where a router connects to a network. A router gives and receives data through its interfaces. These are also used as part of the routing table 接口：对于路由器而言，接口指的是路由器与网络相连的端口。路由器通过这些接口来发送和接收数据。这些接口也是路由表的重要组成部分。
Interior gateway: Interior gateway protocols are used by routers to share information within a single autonomous system 内部网关：内部网关协议被路由器用来在同一个自治系统内部共享信息。
Internet Protocol (IP): The most common protocol used in the network layer 互联网协议（IP）：网络层中最常用的协议
Internet Service Provider (ISP): A company that provides a consumer an internet connection 互联网服务提供商（ISP）：即为消费者提供互联网连接服务的公司。
Internetwork: A collection of networks connected together through routers - the most famous of these being the Internet 互联网：由通过路由器相互连接的各种网络构成的网络体系——其中最著名的就是互联网。
IP datagram: A highly structured series of fields that are strictly defined IP 数据报：由一系列结构严谨、定义明确的字段构成的数据结构
IP masquerading: The NAT obscures the sender's IP address from the receiver IP 地址伪装：NAT 技术会隐藏发送方的 IP 地址，使其无法被接收方识别。
IP options field: An optional field and is used to set special characteristics for datagrams primarily used for testing purposes IP 选项字段：这是一个可选字段，用于为那些主要用于测试目的的数据报设置特殊属性。
L
Line coding: Modulation used for computer networks 线路编码：用于计算机网络的调制方式
Listen: It means that a TCP socket is ready and listening for incoming connections 请注意：这意味着该 TCP 套接字已经准备好，正在等待外部连接的建立。
Local Area Network (LAN): A single network in which multiple devices are connected 局域网（LAN）：由多台设备连接而成的单一网络。
M
MAC filtering: Access points are configured to only allow for connections from a specific set of MAC addresses belonging to devices you trust MAC 地址过滤：接入点被设置为只允许来自那些属于您所信任的设备的特定 MAC 地址的连接请求。
MAC(Media Access Control) address: A globally unique identifier attached to an individual network interface. It's a 48-bit number normally represented by six groupings of two hexadecimal numbers MAC 地址：这是一种分配给每个网络接口的全球唯一标识符。它是一个 48 位的数字，通常由六组各两位的十六进制数字表示。
Mesh networks: Like ad-hoc networks, lots of devices communicate with each other device, forming a mesh if you were to draw lines for all the links between all the nodes 网状网络：与自组织网络类似，许多设备相互进行通信。如果把所有节点之间的连接线都画出来，就会形成一个网状结构。
Metered connection: An internet connection where all data transfer usage is tracked. Cell phone plans that have a limit on data usage per month or that charge based on usage are examples of metered connections 计量式连接：这是一种对所有数据传输量进行计量的互联网连接方式。那些对每月的数据使用量有限制，或者按实际使用量收费的手机套餐，都属于计量式连接的例子。
Modulation: A way of varying the voltage of a constant electrical charge moving across a standard copper network cable 调制：一种改变在标准铜缆中传输的恒定电荷的电压的方法。
Multicast frame: If the least significant bit in the first octet of a destination address is set to one, it means you're dealing with a multicast frame. A multicast frame is similarly set to all devices on the local network signal, and it will be accepted or discarded by each device depending on criteria aside from their own hardware MAC address 多播帧：如果目标地址的第一个八位组中的最低位为 1，那就说明这是一个多播帧。多播帧会被发送到局域网中的所有设备。各设备会根据自身硬件 MAC 地址等因素来决定是否接收该多播帧。
Multiplexing: It means that nodes on the network have the ability to direct traffic toward many different receiving services 多路复用：这意味着网络中的节点能够将数据流量导向多个不同的接收端。
MX record: It stands for mail exchange and this resource record is used in order to deliver email to the correct server MX 记录：其全称为“邮件交换记录”。这种资源记录的作用是确保邮件能够被正确地发送到目标服务器上。
N
Name resolution: This process of using DNS to turn a domain name into an IP address 名称解析：即利用 DNS 将域名转换为 IP 地址的流程。
Network Address Translation (NAT): A mitigation tool that lets organizations use one public IP address and many private IP addresses within the network 网络地址转换（NAT）：这是一种缓解措施，允许组织在网络中使用一个公共 IP 地址以及多个私有 IP 地址。
Network layer: It's the layer that allows different networks to communicate with each other through devices known as routers. It is responsible for getting data delivered across a collection of networks 网络层：这一层的作用是让不同的网络能够通过被称为路由器的设备相互通信。它负责确保数据能够在各种网络之间顺利传输。
Network port: The physical connector to be able to connect a device to the network. This may be attached directly to a device on a computer network, or could also be located on a wall or on a patch panel 网络端口：用于将设备连接到网络的物理接口。该端口可以直接连接到计算机网络中的设备上，也可以安装在墙上或配线架上。
Network switch: It is a level 2 or data link device that can connect to many devices so they can communicate. It can inspect the contents of the Ethernet protocol data being sent around the network, determine which system the data is intended for and then only send that data to that one system 网络交换机：这是一种第二层或数据链路层设备，能够将多台设备连接起来，从而使它们能够相互通信。网络交换机可以查看在网络中传输的以太网协议数据的内容，确定这些数据是发送给哪个设备的，然后再将数据仅发送给该设备。
Next hop: The IP address of the next router that should receive data intended for the destination networking question or this could just state the network is directly connected and that there aren't any additional hops needed. Defined as part of the routing table 下一跳：即下一个应接收发往目标网络的数据的路由器的 IP 地址。或者，也可以直接说明该网络是直接相连的，无需经过额外的路由步骤。这一信息是路由表中的一部分。
Node: Any device connected to a network. On most networks, each node will typically act as a server or a client 节点：指连接到网络的任何设备。在大多数网络中，每个节点通常要么充当服务器的角色，要么充当客户端的角色。
Non-metered connection: A connection where your data usage is not tracked or limited, instead you are charged a flat fee for unlimited and unrestricted usage. A Wi-Fi connection is an example of a non-metered connection 非计量式连接：这种连接方式下，用户的流量使用情况不会被记录或限制。用户只需支付固定的费用，即可享受无限、无限制的流量使用。Wi-Fi 连接就是非计量式连接的一个例子。
Non-routable address space: They are ranges of IPs set aside for use by anyone that cannot be routed to 不可路由的地址空间：这些是专门留出来供那些无法被路由到的设备或用户使用的 IP 地址范围。
NS record: It indicates other name servers that may also be responsible for a particular zone NS 记录：它指明了可能也负责某个特定域名的其他名称服务器。
NTP servers: Used to keep all computers on a network synchronized in time NTP 服务器：用于确保网络中的所有计算机在时间上保持同步。
O
Octet: Any number that can be represented by 8 bits 八位数：任何可以用 8 位二进制数来表示的数字
Optical Network Terminator: Converts data from protocols the fiber network can understand to those that are more traditional twisted pair copper networks can understand 光网络终端设备：能够将光纤网络能够理解的数据格式转换为传统的双绞线铜缆网络所能理解的数据格式。
Options field: It is sometimes used for more complicated flow control protocols 选项字段：该字段有时被用于更复杂的流量控制协议中。
Organizationally Unique Identifier (OUI): The first three octets of a MAC address 组织唯一标识符（OUI）：MAC 地址的前三个八位组
OSI model: A model used to define how network devices communicate. This model has seven layers that stack on top of each other: Physical, Data Link, Network, Transport, Session, Presentation, and Application OSI 模型：这是一种用于描述网络设备之间如何进行通信的模型。该模型由七层结构组成，这些层次依次叠加在一起：物理层、数据链路层、网络层、传输层、会话层、表示层和应用层。
P
Padding field: A series of zeros used to ensure the header is the correct total size 填充字段：由一系列零组成的数据，用于确保标题部分的整体大小符合要求。
Pairing: When a wireless peripheral connects to a mobile device, and the two devices exchange information, sometimes including a PIN or password, so that they can remember each other 配对过程：当无线外设连接到移动设备时，这两台设备会互相交换信息，这些信息可能包括 PIN 码或密码。通过这种方式，两台设备就能“记住”彼此了。
Patch panel: A device containing many physical network ports 配线架：一种包含多个物理网络端口的设备
Payload: The actual data being transported, which is everything that isn't a header 有效载荷：即实际被传输的数据，也就是所有不属于标题部分的内容。
Physical layer: It represents the physical devices that interconnect computers 物理层：它指的是用于连接各种计算机的物理设备。
Point-To-Point VPN: Establishes a VPN tunnel between two sites but VPN tunneling logic is handled by network devices at either side, so that users don't all have to establish their own connections 点对点 VPN：在两个站点之间建立 VPN 隧道。不过，VPN 隧道的建立工作由各端的网络设备来处理，因此用户无需自行建立连接。
Pointer resource record: It resolves an IP to a name 指针资源记录：用于将 IP 地址解析为对应的域名。
Port forwarding: A technique where specific destination ports can be configured to always be delivered to specific nodes 端口转发：这是一种配置方式，通过该方式，可以确保特定的目标端口始终被发送到预定的节点上。
Port preservation: A technique where the source port chosen by a client, is the same port used by the router 端口保留：这是一种技术，其中客户端所选择的源端口，与路由器所使用的端口相同。
Port: It is a 16-bit number that's used to direct traffic to specific services running on a networked computer 端口：这是一个 16 位数字，用于将网络流量引导至网络计算机上运行的特定服务。
Preamble: The first part of an Ethernet frame, it is 8 bytes or 64 bits long and can itself be split into two sections 前导码：以太网帧的第一部分，长度为 8 字节或 64 位。该前导码本身又可进一步划分为两个部分。
Presentation layer: It is responsible for making sure that the unencapsulated application layer data is actually able to be understood by the application in question 表示层：其职责是确保那些未经封装的应用层数据能够被相应的应用程序所理解。
Protocol field: A protocol field is an 8-bit field that contains data about what transport layer protocol is being used 协议字段：协议字段是一个 8 位长的字段，用于存储与所使用的传输层协议相关的信息。
Protocol: A defined set of standards that computers must follow in order to communicate properly is called a protocol 协议：所谓协议，就是计算机为能够正常进行通信而必须遵循的一套既定标准。
Proxy service: A server that acts on behalf of a client in order to access another service 代理服务：一种代表客户端来访问其他服务的服务器。
PSH flag: One of the TCP control flags. PSH is short for push. This flag means that the transmitting device wants the receiving device to push currently- buffered data to the application on the receiving end as soon as possible PSH 标志：这是 TCP 控制标志之一。PSH 是“push”的缩写。该标志表示：发送方希望接收方能尽快将当前缓存中的数据传递给接收端的应用程序。
Q
Quad A (AAAA) record: It is very similar to an A record except that it returns in IPv6 address instead of an IPv4 address Quad A（AAAA）记录：它与 A 记录非常相似，只不过它返回的是 IPv6 地址，而不是 IPv4 地址。
R
Receiving address: The MAC address of the access point that should receive the frame 接收地址：应接收该数据帧的接入点的 MAC 地址
Recursive name servers: Servers that perform full DNS resolution requests 递归名称服务器：负责处理所有 DNS 解析请求的服务器
Reverse lookup zone files: They let DNS resolvers ask for an IP, and get the FQDN associated with it returned 反向查找区域文件：它们允许 DNS 解析器查询某个 IP 地址，从而获取与该 IP 地址对应的完整域名。
Reverse proxy: A service that might appear to be a single server to external clients, but actually represents many servers living behind it 反向代理：对外部客户端来说，它看起来就像是一台服务器，但实际上，它背后隐藏着多台服务器。
Round robin: It is a concept that involves iterating over a list of items one by one in an orderly fashion 循环处理：这是一种按顺序依次处理列表中每个元素的机制。
Router: A device that knows how to forward data between independent networks 路由器：一种能够知道如何在各个独立网络之间转发数据的设备。
Routing protocols: Special protocols the routers use to speak to each other in order to share what information they might have 路由协议：路由器之间用来互相通信的专用协议，通过这些协议，路由器可以共享彼此所拥有的信息。
RST flag: One of the TCP control flags. RST is short for reset. This flag means that one of the sides in a TCP connection hasn't been able to properly recover from a series of missing or malformed segments RST 标志：这是 TCP 控制标志之一。RST 是“reset”的缩写。该标志表示，在 TCP 连接中，有一方未能正确地从一系列丢失或格式错误的数据包中恢复过来。
S
Sequence control field: A field that is 16 bits long and mainly contains a sequence number used to keep track of ordering the frames 序列控制字段：这是一个长度为 16 位的字段，主要用于存储用于确定帧的顺序的序列号。
Sequence number: A 32-bit number that's used to keep track of where in a sequence of TCP segments this one is expected to be 序列号：这是一个 32 位的数字，用于确定该 TCP 数据段在整组 TCP 数据段中的位置。
Server or Service: A program running on a computer waiting to be asked for data 服务器或服务：在计算机上运行的程序，随时准备根据请求提供数据。
Server: A device that provides data to another device that is requesting that data, also known as a client 服务器：一种向请求数据的设备提供数据的设备。请求数据的设备则被称为客户端。
Service type field: A eight bit field that can be used to specify details about quality of service or QoS technologies 服务类型字段：这是一个 8 位字段，可用于指定与服务质量或 QoS 技术相关的详细信息。
Session layer: The network layer responsible for facilitating the communication between actual applications and the transport layer 会话层：负责协调各个应用程序与传输层之间的通信的网络层。
Short-range wireless network: It is what mobile devices uses to connect to their peripherals 短距离无线网络：这就是移动设备用来与外部设备进行连接的机制。
Simplex communication: A form of data communication that only goes in one direction across a cable 单工通信：一种仅允许数据沿电缆单向传输的数据通信方式。
Socket: The instantiation of an endpoint in a potential TCP connection 套接字：在潜在的 TCP 连接中，端点的具体实现/表现形式
Source MAC address: The hardware address of the device that sent the ethernet frame or data packet. In the data packet it follows the destination MAC address 源 MAC 地址：发送该以太网帧或数据包的设备的硬件地址。在数据包中，源 MAC 地址位于目的 MAC 地址之后。
Source port: A high numbered port chosen from a special section of ports known as ephemeral ports 源端口：从被称为“临时端口”的特殊端口范围内选取的、编号较高的端口。
SRV record: A service record used to define the location of various specific services SRV 记录：一种用于指定各类特定服务所在位置的服务记录。
Start Frame Delimiter (SFD): The last byte in the preamble, that signals to a receiving device that the preamble is over and that the actual frame contents will now follow 起始帧分隔符（SFD）：位于前导码的最后一个字节。该字节向接收设备表明，前导码已经结束，接下来就是实际的帧数据了。
Start of authority: A declaration of the zone and the name of the name server that is authoritative for it 授权起始点：对该区域的说明，以及负责该区域的名称服务器的名称。
Static IP address: An IP address that must be manually configured on a node 静态 IP 地址：必须在节点上手动配置的 IP 地址。
Subnet mask: 32-bit numbers that are normally written as four octets of decimal numbers 子网掩码：32 位的数字，通常表示为四个十进制数。
Subnetting: The process of taking a large network and splitting it up into many individual smaller sub networks or subnets 子网划分：将一个大型网络拆分成多个较小的子网络的过程。
Symmetric Digital Subscriber Line (SDSL): A device that establishes data connections across phone lines and has upload and download speeds that are the same 对称数字用户线（SDSL）：一种通过电话线来建立数据连接的设备，其上传和下载速度是相同的。
SYN flag: One of the TCP flags. SYN stands for synchronize. This flag is used when first establishing a TCP connection and make sure the receiving end knows to examine the sequence number field SYN 标志：这是 TCP 标志之一。SYN 代表“同步”。该标志在建立 TCP 连接时使用，确保接收端能够正确解析序列号字段。
SYN_RECEIVED: A TCP socket state that means that a socket previously in a listener state, has received a synchronization request and sent a SYN_ACK back SYN_RECEIVED：这是一种 TCP 套接字的状态，表示原本处于监听状态的套接字已经收到了同步请求，并返回了 SYN_ACK 响应。
SYN_SENT: A TCP socket state that means that a synchronization request has been sent, but the connection hasn't been established yet SYN_SENT：这是一种 TCP 套接字状态，表示已经发送了同步请求，但连接尚未建立。
T
T-Carrier technologies: Technologies Invented to transmit multiple phone calls over a single link. Eventually, they also became common transmission systems to transfer data much faster than any dial-up connection could handle T-Carrier 技术：这是一种旨在通过单一连接来传输多个电话通话的技术。后来，这种技术也被广泛用于数据传输领域，其传输速度远远高于普通的拨号连接方式。
TCP checksum: A mechanism that makes sure that no data is lost or corrupted during a transfer TCP 校验和：一种确保在数据传输过程中不会丢失或损坏数据的机制。
TCP segment: A payload section of an IP datagram made up of a TCP header and a data section TCP 数据段：IP 数据报中的有效载荷部分，由 TCP 头部和数据部分组成。
TCP window: The range of sequence numbers that might be sent before an acknowledgement is required TCP 窗口：在需要确认之前可以发送的序列号范围
Time-To-Live field (TTL): An 8-bit field that indicates how many router hops a datagram can traverse before it's thrown away 生存时间字段（TTL）：这是一个 8 位字段，用于指示数据包在被丢弃之前，可以在多少个路由器上被转发。
Time-To-Live field (TTL): An 8-bit field that indicates how many router hops a datagram can traverse before it's thrown away 生存时间字段（TTL）：这是一个 8 位字段，用于指示数据包在被丢弃之前，可以在多少个路由器上被转发。
Top Level Domain (TLD): The top level of the DNS or the last part of a domain name. For example, the “com” in www.weather.com 顶级域名（TLD）：DNS 结构的最高层级，也就是域名的最后一部分。例如，在 www.weather.com 中，“com”就是顶级域名。
Total hops: The total number of devices data passes through to get from its source to its destination. Routers try to choose the shortest path, so fewest hops possible. The routing table is used to keep track of this 总跳数：数据从源点传输到目的地过程中所经过的设备的总数。路由器会尽量选择最短的路径，从而减少跳数。路由表则用于记录这些信息。
Total length field: A 16-bit field that indicates the total length of the IP datagram it's attached to 总长度字段：这是一个 16 位的字段，用于表示其所对应的 IP 数据报的总长度。
Transmission Control Protocol (TCP): The data transfer protocol most commonly used in the fourth layer. This protocol requires an established connection between the client and server 传输控制协议（TCP）：这是第四层中最常用的数据传输协议。该协议要求客户端与服务器之间必须先建立连接才能进行数据传输。
Transmitter address: The MAC address of whatever has just transmitted the frame 发送方地址：刚刚发送该数据帧的设备的 MAC 地址
Transport layer: The network layer that sorts out which client and server programs are supposed to get the data 传输层：负责确定哪些客户端和服务器程序应该接收这些数据的网络层。
Twisted pair cable: The most common type of cabling used for connecting computing devices. It features pairs of copper wires that are twisted together 双绞线：这是一种最常用的连接计算设备的电缆。其特点是由多对相互绞合在一起的铜线构成的。
Two-factor authentication: A technique where more than just a username and password are required to authenticate. Usually, a short-lived numerical token is generated by the user through a specialized piece of hardware or software 双重身份验证：这是一种需要用户提供用户名和密码之外的其他信息才能完成身份验证的机制。通常，用户会通过专门的硬件或软件来生成一个临时性的数字令牌，以此作为身份验证的依据。
TXT record: It stands for text and was originally intended to be used only for associating some descriptive text with a domain name for human consumption TXT 记录：其名称中的“Text”意为文本。最初，TXT 记录的作用仅仅是将一些描述性文本与域名关联起来，以便人们能够理解该域名的含义。
Types of DNS servers: There are five primary types of DNS servers; caching name servers, recursive name servers, root name servers, TLD name servers, and authoritative name servers DNS 服务器的类型：DNS 服务器主要分为五种类型：缓存域名服务器、递归域名服务器、根域名服务器、顶级域名服务器以及权威域名服务器。
U
Unicast transmission: A unicast transmission is always meant for just one receiving address 单播传输：单播传输始终是针对唯一的接收地址进行的。
URG flag: One of the TCP control flags. URG is short for urgent. A value of one here indicates that the segment is considered urgent and that the urgent pointer field has more data about this URG 标志位：这是 TCP 控制标志位之一。URG 是“urgent”的缩写。当该标志位的值为 1 时，表示该数据段属于紧急数据，且“紧急指针”字段中还包含与该数据段相关的更多信息。
Urgent pointer field: A field used in conjunction with one of the TCP control flags to point out particular segments that might be more important than others 紧急指针字段：这是一个与 TCP 控制位之一共同使用的字段，用于标识那些可能比其他数据段更为重要的数据段。
User Datagram Protocol (UDP): A transfer protocol that does not rely on connections. This protocol does not support the concept of an acknowledgement. With UDP, you just set a destination port and send the data packet 用户数据报协议（UDP）：这是一种不依赖连接层的传输协议。该协议不支持确认机制。使用 UDP 时，只需指定目标端口，然后发送数据包即可。
V
Virtual LAN (VLAN): It is a technique that lets you have multiple logical LANs operating on the same physical equipment 虚拟局域网（VLAN）：这是一种技术，允许在相同的物理设备上运行多个逻辑局域网。
Virtual Private Network (VPN): A technology that allows for the extension of a private or local network, to a host that might not work on that same local network 虚拟专用网络（VPN）：这是一种能够将私有网络或局域网扩展到那些无法接入该局域网的设备上的技术。
VLAN header: A piece of data that indicates what the frame itself is. In a data packet it is followed by the EtherType VLAN 头部：这是一段用于指示数据帧具体类型的数据。在数据包中，VLAN 头部之后就是 EtherType 字段。
W
Wi-Fi Protected Access (WPA): A security program that uses a 128-bit key to protect wireless computer networks, which makes it more difficult to crack than WEP Wi-Fi 保护访问协议（WPA）：这是一种安全机制，它使用 128 位密钥来保护无线计算机网络。这样一来，要破解该加密机制就比使用 WEP 协议要困难得多。
Wide area network: Acts like a single network but spans across multiple physical locations. WAN technologies usually require that you contract a link across the Internet with your ISP 广域网：虽然表现为一个整体网络，但实际上覆盖了多个物理位置。使用广域网技术时，通常需要通过互联网与互联网服务提供商签订连接协议。
Wired Equivalence Privacy (WEP): An encryption technology that provides a very low level of privacy. WEP should really only be seen as being as safe as sending unencrypted data over a wired connection 有线等效隐私机制（WEP）：这是一种加密技术，但其提供的隐私保护水平相当低。实际上，WEP 的安全性几乎等同于通过有线连接传输未加密的数据。
Wireless access point: A device that bridges the wireless and wired portions of a network 无线接入点：一种用于连接网络中的无线部分和有线部分的设备。
Go through all the questions at your own pace. Once you've reviewed everything, you'll see a final question asking if you've completed the guide. Clicking "Yes" will let us know you've finished, and you'll get credit for your work. 请按照自己的节奏来回答所有问题。当您完成所有问题的回答后，会看到最后一个问题，询问您是否已经完成了整个流程。点击“是”即可表示您已完成任务，您的努力也将得到认可。
Question 1 问题 1
The concept of taking traffic that’s all aimed at the same node and delivering it to the proper receiving service is known as . 将所有指向同一节点的流量重新导向到相应的接收服务端这一概念，被称为。
A. demultiplexing A. 解复用
B. multiplexing B. 多路复用
C. encapsulation C. 封装
D. routing D. 路由选择
Answer: A. demultiplexing 答案：A. 解复用
Multiplexing is the process of combining multiple data streams into a single signal for transmission over a shared medium. Encapsulation involves adding control information (headers and trailers) to data as it moves through network layers. Routing, typically at the network layer, determines the best path for data packets to travel across networks to a destination host. However, the specific process of taking this combined traffic, once it arrives at the intended node, and separating it to deliver it to the correct receiving application or service (often identified by port numbers) is called demultiplexing 多路复用是一种将多个数据流合并为单一信号，以便通过共享介质进行传输的技术。封装则是指在数据在网络各层中传输时，为其添加控制信息（如头部和尾部信息）。路由功能通常在网络层实现，它负责确定数据包在网络中传输到目标主机的最佳路径。而当这些合并后的数据包到达目标节点后，需要将它们拆分开来，以便将数据传递给正确的接收方或服务程序——这一过程被称为多路分解。.
Question 2 问题 2
Which field in the TCP header provides the information for the receiving network device to understand where the actual data payload begins?
在 TCP 头部中，哪个字段能够为接收端网络设备提供信息，从而让其知道实际的数据内容从何处开始？
A. Checksum A. 校验和
B. Acknowledgement number B. 承认/认可编号
C. Data offset C. 数据偏移量
D. Sequence number D. 序列号
Answer: D. Data offset 答案：D. 数据偏移量
The Checksum field is used for error detection, ensuring the integrity of the TCP segment. The Acknowledgment number confirms the receipt of data from the other end of the connection, while the Sequence number indicates the order of bytes in the stream. None of these fields directly specify the start of the data payload. Instead, the TCP header can be of variable length due to the optional "options" field. To inform the receiving device precisely where the fixed-size header ends and the actual application data begins, the TCP header includes a specific field. This field is the Data offset 校验和字段用于错误检测，从而确保 TCP 数据段的完整性。确认号则用于确认从连接另一端接收到的数据。序列号则用于指示数据流中各字节的排列顺序。这些字段均无法直接指示数据内容的起始位置。不过，由于存在可选的“选项”字段，TCP 头部可以具有可变的长度。为了让接收端能够准确知道固定长度的头部何时结束，以及实际的应用数据从何处开始，TCP 头部包含了一个专门的字段——数据偏移量。.
Question 3 问题 3
Which field in a Transmission Control Protocol (TCP) header provides the next expected segment? 在传输控制协议（TCP）的头部中，哪个字段用于指示下一个即将传输的数据段？
A. Acknowledgement number A. 承认/确认编号
B. Sequence number B. 序列号
C. Data offset C. 数据偏移量
D. Checksum D. 校验和
Answer: A. Acknowledgement number 答案：A. 确认编号
The Sequence number identifies the position of the first byte of data in the current segment being sent, while the Data offset specifies the length of the TCP header, indicating where the actual data payload begins. The Checksum field is used for error detection to ensure data integrity. However, to inform the sending host which byte of data it should transmit next to continue the flow of communication, TCP utilizes a specific field that indicates the sequence number of the next byte the receiver expects to get. This field is the Acknowledgement number 序列号用于标识当前正在传输的数据段中，第一个数据字节的位置。而数据偏移量则用于指示 TCP 头部的长度，从而确定实际数据部分的起始位置。校验和字段则用于错误检测，以确保数据的完整性。不过，为了让发送方知道接下来应该传输哪个数据字节以维持通信的连续性，TCP 使用了一个专门的字段来指示接收方期望接收的下一个数据的序列号。这个字段就是确认号。.
Question 4 问题 4
The sequence of SYN, SYN/ACK, and ACK packets is known as the . SYN、SYN/ACK 和 ACK 数据包的这种顺序被称为。
A. four-way handshake A. 四方握手协议
B. two-way handshake B. 双向握手机制
C. high five C. 拍手庆祝
D. three-way handshake D. 三次握手机制
Answer: D. three-way handshake 答案：D. 三次握手
The initial exchange of packets to establish a TCP connection involves the client sending a SYN (synchronize) packet, the server responding with a SYN/ACK (synchronize-acknowledgment) packet, and finally the client sending an ACK (acknowledgment) packet. This sequence ensures that both sides agree on initial sequence numbers and are ready to send and receive data reliably. While a "four-way handshake" exists, it refers to the process of terminating a TCP connection. Therefore, the sequence of SYN, SYN/ACK, and ACK packets is known as the three-way handshake 在建立 TCP 连接的过程中，数据包的交换分为几个步骤：首先，客户端发送 SYN 数据包；接着，服务器回复 SYN/ACK 数据包；最后，客户端再发送 ACK 数据包。这一系列操作确保了双方能够就初始序列号达成一致，从而能够可靠地发送和接收数据。虽然也有一种“四次握手”的说法，但实际上那是用于终止 TCP 连接的流程。因此，SYN、SYN/ACK 和 ACK 数据包的交换过程被称为“三次握手”。.
Question 5 问题 5
A Transmission Control Protocol (TCP) connection is in working order and both sides can send each other data. What is the TCP socket state? 传输控制协议（TCP）连接正常，双方能够互相发送数据。那么，TCP 套接字的状态如何呢？
A. ESTABLISHED A. 已建立/已设立
B. LISTEN B. 听着/请倾听
C. SYN_SENT
D. SYN_RECEIVED D. 接收到的 SYN
Answer: A. ESTABLISHED 答案：A. 已确立/已建立
The LISTEN state indicates that a server socket is waiting for incoming connection requests. SYN_SENT means a client has sent a SYN packet and is awaiting a SYN/ACK. SYN_RECEIVED signifies that a server has received a SYN and sent a SYN/ACK, now waiting for the client's final ACK. When a TCP connection has successfully completed the three-way handshake and is fully established, allowing both sides to actively exchange data, the socket is in the ESTABLISHED state. “LISTEN”状态表示服务器套接字正在等待来自客户端的连接请求。“SYN_SENT”状态意味着客户端已发送了 SYN 数据包，现在正在等待 SYN/ACK 响应。“SYN_RECEIVED”状态则表示服务器已收到 SYN 数据包并发送了 SYN/ACK 响应，现在正在等待客户端的最终 ACK 响应。当 TCP 连接成功完成三次握手过程并正式建立之后，双方就可以开始互相传输数据了。此时，该套接字处于“ESTABLISHED”状态。
Question 6 问题 6
If the checksum doesn't compute for a packet sent at the Internet Protocol (IP) level, what will happen to the data? 如果在互联网协议层发送的数据包无法计算出校验和，那么这些数据会怎么样呢？
A. The data will be sent back to the sending node with an error. A. 这些数据会带着错误被发送回发送节点。
B. The data will be discarded B. 这些数据将被丢弃。
C. The data will be resent C. 数据将被重新发送。
D. It will be sent, but may be out of order D. 产品会被寄出，但有可能存在故障/无法正常使用。
Answer: B. The data will be discarded 答案：B。这些数据将被丢弃。
The Internet Protocol (IP) is designed as a best-effort, connectionless protocol, meaning it doesn't guarantee delivery or provide error recovery mechanisms like retransmission. Its primary role is to get packets from a source to a destination. If the IP header's checksum fails to compute correctly, it indicates that the header itself has been corrupted during transit. Because the IP layer cannot trust the corrupted header information (which contains critical routing details like the destination address), it does not send the data back for correction, nor does it attempt to retransmit it. Instead, to prevent misrouting or further network issues from corrupting data, the data will be discarded 互联网协议（IP）本质上是一种“尽力而为”的、无连接型的协议。这意味着它不保证数据的成功传输，也不提供诸如重传之类的错误恢复机制。IP 协议的主要功能就是将数据包从源地址传输到目标地址。如果 IP 协议的头部校验和计算结果不正确，那就说明该头部在传输过程中已被损坏。由于 IP 层无法信任这些被损坏的头部信息（而这些信息中包含着诸如目标地址之类的重要路由信息），因此它既不会尝试重新发送数据，也不会尝试对数据进行修复。为了避免因数据损坏而导致的路由错误或进一步的网络问题，这些数据会被直接丢弃。.
Question 7 问题 7
A communication sent through Transmission Control Protocol (TCP) arrives out of order. What allows the data to be put back together in the correct order?
通过传输控制协议（TCP）发送的数据往往会出现顺序混乱的情况。那么，究竟是什么机制使得数据能够被重新整理成正确的顺序呢？
A. Acknowledgement number A. 承认/确认编号
B. Preamble B. 序言/前言
C. Checksum C. 校验和
D. Sequence numbers D. 序列号
Answer: D. Sequence numbers 答案：D. 序列号
The Acknowledgement number is used by the receiver to inform the sender of the next byte it expects, ensuring reliable flow but not directly reordering. The Preamble is a synchronization pattern found at the beginning of an Ethernet frame, unrelated to TCP segment ordering. The Checksum is used for error detection to verify the integrity of the segment. However, to reconstruct data that might arrive out of sequence, each TCP segment is assigned a unique identifier indicating the order of the bytes it contains. This crucial information allows the receiving TCP stack to correctly reassemble the data stream, even if segments arrive non-sequentially. This is achieved through sequence numbers 确认号被接收方用来告知发送方它期望接收的下一个字节是什么。这样一来，就能确保数据传输的可靠性，不过确认号本身并不直接负责数据的重新排序。前导码是位于以太网帧开头的同步信号，与 TCP 数据段的排序无关。校验和则用于检测错误，从而确保数据段的完整性。不过，为了能够重新组装那些可能以乱序方式到达的数据，每个 TCP 数据段都被赋予了唯一的标识符，从而表明其中各字节的顺序。这一关键信息使得接收端的 TCP 栈能够正确地重新组装数据流，即便各数据段是以乱序方式到达的。这一功能是通过序列号来实现的。.
Question 8 问题 8
In the OSI network model, the ________ is responsible for facilitating the communication between actual applications and the transport layer. 在 OSI 网络模型中，________负责促进实际应用程序与传输层之间的通信。
A. presentation layer A. 表现层
B. physical layer B. 物理层
C. application layer C. 应用层
D. session layer D. 会话层
Answer: D. session layer 答案：D. 会话层
The physical layer is concerned with the transmission of raw bits, while the presentation layer focuses on data formatting, encryption, and compression to ensure data is understandable. The application layer provides direct network services to end-user applications. However, the layer specifically tasked with establishing, managing, and terminating the communication dialogue or "session" between two applications, thereby orchestrating their interaction and ensuring proper synchronization before handing off to or receiving from the transport layer, is the session layer 物理层负责原始比特数据的传输，而表示层则负责数据的格式化、加密和压缩处理，以确保数据能够被正确理解。应用层则直接向最终用户的应用程序提供网络服务。不过，专门负责在两个应用程序之间建立、管理和终止通信连接或“会话”的层，就是会话层。会话层负责协调这两个应用程序之间的交互，确保在数据被传递给传输层或从传输层接收数据之前，一切都能正常同步。.
Question 9 问题 9
Which Transmission Control Protocol (TCP) flag is used to make sure the receiving end knows to examine the sequence number field?
在传输控制协议（TCP）中，究竟是哪个标志位被用来确保接收端能够正确地查看序列号字段呢？
A. FIN A. 财务/金融
B. SYN B. SYN 请求
C. PSH
D. RST
Answer: B. SYN 答案：B. SYN
The FIN flag is used to gracefully close a connection, while PSH requests that data be immediately delivered to the application. The RST flag is used to abruptly terminate a connection, often due to an error. However, during the initial phase of connection establishment, a specific flag is sent to synchronize sequence numbers between the sender and receiver, indicating that the receiving end should examine the sequence number field. This flag is the SYN flag. FIN 标志用于优雅地关闭连接，而 PSH 标志则要求数据能立即被传送给应用程序。RST 标志则用于强制终止连接，通常是由于出现了错误。不过，在连接建立的初始阶段，会发送一个特殊的标志来同步发送方和接收方的序列号，从而确保接收端会仔细检查序列号字段。这个特殊的标志就是 SYN 标志。
Question 10 问题 10
You are sending a very small amount of information that you need the listening program to respond to immediately. Which Transmission Control Protocol (TCP) flag will be used? 你发送的信息量非常少，因此需要让监听程序能够立即作出响应。那么，应该使用哪种传输控制协议（TCP）标志呢？
A. ACK A. 已确认/收到
B. URG B. 紧急/刻不容缓
C. PSH
D. RST
Answer: C. PSH 答案：C. PSH
The ACK flag is used to acknowledge received data, while the RST flag is used to immediately terminate a connection due to an error. The URG flag indicates that urgent data is present and should be processed quickly, potentially out of normal sequence. However, when sending a small amount of information that requires the receiving application to process and respond without delay, bypassing any buffering delays at the TCP layer, a specific flag is set to instruct the receiver's TCP stack to deliver the data to the application immediately. This flag is the PSH flag. ACK 标志用于确认已接收到的数据，而 RST 标志则用于在出现错误时立即终止连接。URG 标志表示有紧急数据需要立即处理，可能无需遵循正常的处理顺序。不过，当需要发送少量信息，且接收方应用程序必须立即处理并响应这些信息时，可以设置专门的标志来指示接收方的 TCP 堆栈立即将数据传递给应用程序。这个标志就是 PSH 标志。
Question 11 问题 11
Application layer data lives in the _____ section of the transport layer protocol. 应用层数据位于传输层协议的_____部分。
A. payload A. 载荷/有效载荷
B. header B. 标题/页眉
C. footer C. 底部信息/页脚内容
D. flags D. 标志/标记
Answer: A. payload 答案：A. 载荷/有效载荷
The header section of a transport layer protocol contains control information necessary for the protocol's operation, such as source and destination ports, sequence numbers, and flags, but not the application data itself. A footer, or trailer, is sometimes found in lower-layer protocols for error checking, but it's not where application data resides in the transport layer. Flags are specific bits within the header that convey control information. The actual data originating from the application layer that is being transported by the TCP or UDP segment is contained within the payload section of the transport layer protocol. 传输层协议的头部包含该协议运行所必需的控制信息，比如源端口、目的端口、序列号以及各种标志位。不过，头部并不包含应用程序数据本身。在较低层的协议中，有时会有用于错误检测的尾部信息，但在传输层中，应用程序数据并不存储在尾部。那些标志位其实是头部中的特定比特，用于传递控制信息。而那些源自应用程序层、并通过 TCP 或 UDP 协议段进行传输的实际数据，则存储在传输层协议的负载部分中。
Question 12 问题 12
What layer of the TCP/IP Five-Layer Network Model allows applications to communicate in a way they understand? 在 TCP/IP 五层网络模型中，是哪一层使得各种应用程序能够以自己能够理解的方式来进行通信呢？
A. application A. 申请/申请表
B. data-link B. 数据链路层
C. transport C. 运输/运输方式
D. network D. 网络
Answer: A. application 答案：A. 申请/提交
The data-link layer is responsible for local network communication and framing, while the network layer handles logical addressing and routing across different networks. The transport layer provides end-to-end communication between processes on hosts, focusing on reliability and flow control, but it doesn't interpret the data for applications. The layer specifically designed to provide services that applications can directly use, enabling them to communicate using protocols and data formats they understand, is the application layer. 数据链路层负责本地网络中的数据传输和数据帧的构建，而网络层则负责不同网络之间的逻辑地址分配和路由选择。传输层则负责主机上各个进程之间的端到端通信，注重数据的可靠性和流量控制，但不负责对数据的解析。应用层则是专门为满足应用程序的需求而设计的：它提供应用程序可以直接使用的服务，使应用程序能够使用自己熟悉的协议和数据格式来进行通信。
Question 13 问题 13
Ports that are generally used to establish outbound connections are known as ______ ports.
通常用于建立出站连接的端口被称为________端口。
A. registered A. 已注册
B. reserved B. 已预订/被保留
C. ephemeral C. 短暂的/短暂的
D. system D. 系统
Answer: C. ephemeral ports 答案：C. 短期使用的端口
Registered ports (1024-49151) and system/well-known ports (0-1023) are typically used by specific services for inbound connections, or for outbound connections from specific client applications. Reserved ports are simply those set aside for future use. However, when a client program initiates an outbound connection, the operating system dynamically assigns a temporary port number from a range of high-numbered ports, ensuring unique identification for that particular connection for its duration. These temporary ports are known as ephemeral ports 注册端口（1024-49151）和系统/已知端口（0-1023）通常被特定的服务用于处理入站连接，或被特定的客户端应用程序用于处理出站连接。而保留端口则是那些被预先留出来以供将来使用的端口。不过，当客户端程序发起出站连接时，操作系统会从较高编号的端口中动态分配一个临时端口号，从而确保该连接的唯一性。这些临时端口被称为“临时端口”。.
Question 14 问题 14
In what order will the Transmission Control Protocol (TCP) generally send all segments? 传输控制协议（TCP）通常会按照什么顺序来发送所有的数据段呢？
A. Random A. 随机
B. Largest to smallest B. 从大到小排列
C. Sequential C. 顺序处理
D. Prioritized D. 优先处理/按优先级排序
Answer: C. Sequential 答案：C. 依次进行/按顺序操作
TCP is a reliable, connection-oriented protocol designed to ensure that data arrives at its destination completely and in the correct order. Sending segments randomly or from largest to smallest would lead to significant reordering issues and make reliable reconstruction of the data nearly impossible. While some applications might prioritize certain data, TCP's fundamental mechanism for ensuring ordered delivery, which is critical for data integrity, involves numbering segments and sending them one after another based on those numbers. Therefore, TCP will generally send all segments in sequential order. TCP 是一种可靠、基于连接的协议，其设计目的就是确保数据能够完整无缺地、按正确的顺序送达目的地。如果随机发送数据段，或者按照从大到小的顺序发送，就会导致严重的数据重排问题，从而使得数据的可靠重建变得几乎不可能。虽然某些应用程序可能会优先处理某些数据，但 TCP 确保数据按顺序传输的机制对于维护数据完整性至关重要。该机制包括给每个数据段编号，然后按照编号顺序依次发送这些数据段。因此，TCP 通常会按顺序发送所有的数据段。
Question 15 问题 15
A communication between two devices is over the maximum limit of an ethernet frame size. The Transmission Control Protocol (TCP) splits up the data into segments. Which field in the header helps keep track of the many segments?
两个设备之间的数据传输量超出了以太网帧大小的极限。传输控制协议（TCP）会将数据分割成多个数据段。在数据包的头部中，哪个字段有助于追踪这些数据段呢？
A. Sequence number A. 序列号
B. Acknowledgement number B. 承认/认可编号
C. Checksum C. 校验和
D. Urgent pointer D. 紧急提示/需要立即处理的事项
Answer: A. Sequence number 答案：A. 序列号
The Acknowledgement number is used by the receiver to inform the sender of the next expected byte, aiding in reliable data flow, but it doesn't primarily track the order of segments being sent. The Checksum is for error detection, ensuring data integrity, not segment ordering. The Urgent pointer indicates urgent data within a segment, which is unrelated to maintaining the order of multiple segments. When a large amount of data needs to be broken down into smaller pieces to fit within network limitations, TCP assigns a unique identifier to the beginning of the data within each piece. This crucial field allows the receiving device to correctly reassemble the segments into their original order, even if they arrive out of sequence. This field is the Sequence number 确认号的作用是让接收方能够告知发送方下一个应接收的字节位置，从而确保数据的可靠传输。不过，确认号并不用于确定各个数据段的发送顺序。校验和则用于检测错误、确保数据完整性，同样与数据段的顺序无关。紧急指针用于标识数据段中的紧急数据，但这与维持多个数据段的正确顺序无关。当需要将大量数据拆分成更小的部分以符合网络传输限制时，TCP 会为每部分数据的起始位置分配一个唯一的标识符。这一关键信息使得接收设备能够即使数据段不是按顺序到达的，也能将其重新组合成原来的顺序。这个标识符就是序列号。.
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