With circuit switching, a (71)path is established between two stations for communication . Switching and transmission resources within the network are (72)for the exclusive use of the circuit for the duration of the connection . The connection is(73)：Once it is established ，it appears to attached devices as if there were a direct connection . Packet switching was designed to provide a more efficient facility than circuit switching for(74)data traffic . Each packet contains some portion of the user data plus control information needed for proper functioning of the network . A key distinguishing element of packet-switching networks is whether the internal operation is datagram or virtual circuit . With internal virtual circuits,a route is defined between two endpoints and all packets for that virtual circuit follow the(75)route, With internal datagrams,each packet is treated independently,and packets intended for the same destination may follow different routes .

 (71)

(75)

 (74)

 (72)

 (73)

With circuit switching, a (71)path is established between two stations for communication . Switching and transmission resources within the network are (72)for the exclusive use of the circuit for the duration of the connection . The connection is(73)：Once it is established ，it appears to attached devices as if there were a direct connection . Packet switching was designed to provide a more efficient facility than circuit switching for(74)data traffic . Each packet contains some portion of the user data plus control information needed for proper functioning of the network . A key distinguishing element of packet-switching networks is whether the internal operation is datagram or virtual circuit . With internal virtual circuits,a route is defined between two endpoints and all packets for that virtual circuit follow the(75)route, With internal datagrams,each packet is treated independently,and packets intended for the same destination may follow different routes .

 (71)

如果DHCP客户端发现分配的IP地址已经被使用，客户端向服务器发出(69)报文，拒绝该IP地址。

在层次化园区网络设计中，(70)是汇聚层的功能。

PGP的功能中不包括(68)。

下面关于路由器的描述中，正确的是(67)。

下图所示的网络拓扑中配置了RIP协议，且RIP协议已更新完成，下表所示为AR2路由器上查看到的路由信息。

从查看到的路由信息可以判断(65)， 造成故障的原因是(66)。