TCP functions at the Open Systems Interconnection(OSI)transport layer, or Layer

TCP functions at the Open Systems Interconnection(OSI)transport layer, or Layer 4. Its chief responsibility is to ensure reliable end-to-end connectivity. IP, located one layer （1）, at the OSI network layer, or Layer 3,communicates the addresses of each packet’s sender and receiver to the routers along the way .Routers and Layer 3 switches can read IP and other Layer 3 protocols. This information, combined with routing tables and other network intelligence, it takes to get across the room or around the world（2）TCP/IP.

The routing process begins with an IP address that is（3）to the sending end station. End stations may be assigned permanent IP addresses or they may borrow them as needed from a Dynamic Host Configuration Protocol(DHCP)server or other service. If the sending end station determines that the destination address is not local, the packet goes to a first-hop router, typically one that is close and has been reassigned to the（4）. The router inspects the packet’s IP address and performs a route table lookup to see if the destination end station resides on the local(physically connected)network, typically called an IP subnet. An IP subnet usually is assigned to each of the router’s network interfaces. If the destination IP address is local, the router searches an internal store of IP addresses and local-device media access control(MAC)addresses. This store is known as the Address Resolution Protocol(ARP) cache. ARP is the universal tool for（5）IP addresses to MAC addresses. If the destination’s MAC address appears, the router installs that MAC address in the packet header(removing its own MAC address because that’s no longer needed)and sends the packet to the destination end station.

（1）A. up

B. down

C. sideward

D. back-fence

（2）A. via

B. through

C. by

D. through out

（3）A. special

B. especial

C. unique

D. particular

（4）A. accepter

B. sender

C. router

D. server

（5）A. searching

B. looking

C. locating

D. matching

B,A,C,B,DTCP完成开放系统互连（OSI）模型中传输层，即第四层的功能。它的主要责任是确保端至端之间的可靠连接。IP位于下一层，在 OSI的网络层，即第三层，把每个包的发送者和接收者地址告诉一路上各个路由器。路由器和第三层交换器可以读出IP和其他的第三层协议。这些信息与路由表以及其他网络智能结合在一起,能通过 TCP/IP在整个房间或围绕地球进行传递。 路由过程始于对发送端站而言是唯一的一个IP地址，端站可以被分配永久的IP地址，或者按需要从动态主机配置协议（DHCP）服务器或其他服务中借用。如果