网络结构在任何网络中,都有一批运行用户程序(也就是应用程序)的机器,我们把这些机器称为主机(host)。这些主机通过通讯子网(Commun:cation subnet)或简称子网连接起来(某些作者使用术语一转送系统或传输系统)。子网的任务是将报文由一台主机传给另一台主机,就像电话系统从发话人到受话人传送话语那样。由于把网络的纯通讯部分(子网)和应用部 Network Architecture In any network, there are a number of machines running user programs (ie, applications), which we call hosts. These hosts are connected via a communi- cation subnet or subnet (some authors use the term transfer system or transmission system). The task of a subnet is to send a message from one host to another, just as a telephone system transmits words from a speaker to a receiver. Due to the pure communication part of the network (subnet) and application department