计算机网络是由多种计算机和各类终端，通过通行线路连接起来组成的一个复合系统 。要实现资源共享、协同处理等网络功能，离不开信息交换，而信息交换必须按照共同的协议进行。 为了减少协议设计的复杂性，大多数网络按层的方式来组织。不同的网络，其层的数量、名称、内容和? Computer network is a complex system composed of a variety of computers and terminals of all kinds, connected by a line of access. To achieve resource sharing, co-processing and other network functions, can not do without information exchange, and information exchange must be in accordance with the common agreement. To reduce the complexity of protocol design, most networks are organized in layers. Different networks, the number of layers, names, contents and?