The classical hybrid model we learn is divided into seven layers, running from the physical layer up through the link, network and transport layers to the application layer.  Also ，session layer and presentation layer. 我们学习的经典的混合模型分为7个层次，从物理层到链路层、网络层和传输层再到应用层。还有，会话层和表示层.

The physical layer specifies how to transmit bits across different kinds of media as electrical (or other analog) signals. 物理层指定如位作为电（或其他模拟）信号在不同类型的媒体上传输。

The link layer is concerned with how to send finite-length messages between directly connected computers with specified levels of reliability.Ethernet and 802.11 are examples of link layer protocols. 链路层涉及如何在具有指定可靠性等级的直接连接的计算机之间发送有限长度的消息，以太网和802.11是链路层协议的例子。

The network layer deals with how to combine multiple links into networks, and networks of networks, into internetworks so that we can send packets between distant computers. This includes the task of finding the path along which to send the packets. IP is the main example protocol we will study for this layer. 网络层处理如多个链路合并到网络中，以及将网络中的网络合并到网络中，以便我们可以在远程计算机之间发送数据包。这包括查找发送数据包的路径的任务。 IP 是我们将要为这一层研究的主要示例协议。

The transport layer strengthens the delivery guarantees of the Network layer, usually with increased reliability, and provide delivery abstractions, such as a reliable byte stream, that match the needs of different applications. TCP is an important example of a transport layer protocol. 传输层增强了网络层的传输保证，通常提高了可靠性，并提供了与不同应用程序的需求相匹配的传输抽象，例如可靠的字节流。 TCP 是传输层协议的一个重要例子。

Finally, the application layer contains programs that make use of the network. Many, but not all, networked applications have user interfaces, such as a Web browser. Our concern, however, is with the portion of the program that uses the network. This is the HTTP protocol in the case of the Web browser. There are also important support programs in the application layer, such as the DNS, that are used by many applications. 最后，应用层包含使用网络的程序。许多（但不是所有）网络应用程序都有用户界面，如web浏览器。然而，我们关心的是使用网络的程序部分。这是web浏览器的http协议。应用层中还有一些重要的支持程序，如DNS，被许多应用程序使用。

Our chapter sequence is based on this model. In this way, we retain the value of the OSI model for understanding network architectures, but concentrate primarily on protocols that are important in practice, from TCP/IP and related protocols to newer ones such as 802.11, SONET, and Bluetooth. The classic hybrid model is divided into five layers, from physical layer to link layer, network layer and transmission layer to application layer. 我们的章节顺序是基于这个经典混合模型的。这样，我们保留了 OSI 模型对于理解网络体系结构的价值，但主要集中在实践中重要的协议上，从 TCP/IP 和相关协议到802.11、sonet和蓝牙等较新的协议。经典的混合模型分为5个层次，从物理层到链路层、网络层和传输层再到应用层。