Networks can be interconnected by different devices. In the physical layer, networks can be connected by (11) 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 (12) ,examine the MAC address, and forward the frames to a different network while doing minor protocol translation in the process. In the network layer, we have routers that can connect two networks. If two networks have (13) network layer, the router may be able to translate between the 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 (14) . As an example, gateways between Internet e-mail and X.400 e-mail must (15) the e-mail message and change various header fields.

A．repeaters

B．relays

C．packages

D．modems

情景交际：补全对话，每空一词。

A: __1__ will happen if we have an English party this evening ?

B: Many students won't __2__, I think.

A: Why?

B: Because we'll have a math test tomorrow morning and we have to __3__ for it,

A: But I called Mr. Wang, our English teacher. He __4__ to come.

B: Give him a call now. And tell him __5__ it.

A: But when is a good time to have __6__?

B: __7__ have it tomorrow evening. And I can help you to organize it well.

A: I'm so __8__ to hear that. What shall l wear to the party, then?

B: You'd __9__ wear the school clothes.

A: What do you think of my new pants? __10__ very cool.

B: I don't think so. School children should wear school uniforms while in school.

A: All right.