● For （71） service, we need a virtual-circuit subnet. Let us see how that works. The

idea behind virtual circuits is to avoid having to choose a new （72） for every packet sent.

Instead, when a connection is established, a route from the （73） machine to the destination

machine is chosen as part of the connection setup and stored in tables inside the （74） . That

route is used for all traffic flowing over the connection, exactly the same way that the

telephone system works. When the connection is released, the virtual circuit is also

terminated. With connection-oriented service, each packet carries an （75） telling which

virtual circuit it belongs to.

（71）A. connectionless B. connection-oriented C. datagram D. telegram

（72）A. processor B. device C. route D. terminal

（73）A. source B. route C. destination D. host

（74）A. connections B. resources C. bridges D. routers

（75）A. address B. identifier C. interface D. element

B、C、A、D、B翻譯：實現(xiàn)面向連接的服務(wù)需要一個虛電路子網(wǎng)，下面看它如何工作。虛電路的主要思想是避免為每一個發(fā)送的分組都進行重新的路由選擇。事實上，在連接建立過程中，從源主機到目標(biāo)主機的路自己經(jīng)選定，并存儲在略由器內(nèi)部的路由表中。連接上的所有通信都使用這個路由，與電話系統(tǒng)的工作方式完全一樣。當(dāng)連接釋放時，虛電路也終止。在面向連接的服務(wù)中，每一個分組都含有一個標(biāo)識符，用于表示該分組所屬的虛電路號。