图 5-1是本章大部分内容的基础。图中展示了 OSI 和 TCP/IP 模型，以及各层的主要协议。集线器、交换机和路由器将两边互连起来。底部的实线表示设备之间的物理连接，也就是我们常说的传输路径。 不过，设备可能会在上层做出决定，因此它们在逻辑上是通过虚线连接的。例如，交换机处理包含 MAC 地址的第 2 层帧。
图 5-1. 型号和设备
桌子和主持人
虽然本章讲述的是网络设备，但如果不提及网络上的主机或节点，本章就不完整。节点拥有 IP 和 MAC 地址，用于在网络基础设施上进行通信。几乎无一例外，设备都使用这些地址来转发数据包或帧。此外，认识到在网络内，几乎所有事情都遵循基于同一套规则的循序渐进的流程也很有帮助。这个过程可以通过一系列表格和数据包来追踪。网络上的所有设备，包括主机，都有这些表，并且能够生成和/或转发数据包。在下面的章节中，我们将讨论主要的设备类型及其用于处理帧的表。表 5-1列出了这些表，以及每个表的用途和在网络中的位置。
节点 A 从 ARP 表中提取适当的 MAC 地址，或发送 ARP 请求并接收适当的应答，从而为第 2 层目标构建帧。
帧从节点 A 通过交换机 1 发送到路由器。
收到帧后，交换机 1 会查询源地址表 (SAT)，以确定目的地的正确端口。
路由器从交换机 1 接收帧，并检查 IP 标头以确定目标网络。
路由器会处理路由表，以确定目的地要使用的正确接口。
路由器从 ARP 表中提取适当的 MAC 地址，或发送 ARP 请求并接收适当的应答，从而为第 2 层目标构建帧。
帧从路由器通过交换机 ...
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