2. 计算机
  3. Networks can be interconnected by different devices in the physical layer networks can be connected by (71) or hubs．which just m

Networks can be interconnected by different devices in the physical layer networks can be connected by (71) or hubs．which just m

admin2010-12-16  49
问题 Networks can be interconnected by different devices in the physical layer networks can be connected by (71) or hubs．which just move the bits from one network to an identical network．One layer up we find bridges and switches which operate at data link layer．They can accept (72) ，examine the MAC address and forward the frames to a different network while doing minor protocol translation in the process in me network layer，we have routers that can connect two networks，If two networks have (73) network layer，the router may be able to translate betweenthe packer formats．In the transport layer we find transport gateway，which can interface between two transport connections Finally，in the application layer，application gateways translate message (74) ．As an example，gateways between Internet e-mail and X，400 e-mail must (75) thee-mail message and change various header fields．
选项 A、analyze
B、parse
C、delete
D、create
答案B
解析 网络可以通过不同的设备互连。在物理层，网络用中继器(reapers)或集线器互连，它们只是在相同的网络之间传送比特串。在上层的数据链路层，可以使用网桥和交换机，它们能够接收数据帧(frames)、检测MAC地址，并可以实现少量的协议转换，把数据帧转发到不同的网络中。在网络层，我们可以使用路由器连接两个网络。如果两个网络有着不同(dissimilar)网络层，路由器能够对分组格式转换。在传输层，我们使用传输网关，它可以在两个传输连接之间建立接口。最后，在应用层，应用网关实现消息语法(syntax)之间的翻译。例如，在Internet邮件和X.400邮件之间的网关必须对邮件报文进行语法分析(parse)，并对报文各个字段做出修改。
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