【正文】 IP的地址也是電線公司分配的地址塊當中的地址。 。DHCP服務器產生一個ACK報文其中包含這個IP地址，也包含DNS服務器的IP（(），默認網關的IP（）。DHCP報文封裝在UDP數據報中，UDP數據報又封裝在IP報文中，IP報文封裝在以太網幀中進行傳輸。以太網幀有一個連接在路由器接口源MAC地址00:22:6B:45:1F:1B和Bob電腦的目的MAC地址00:16:D3:23:68:8A。6. The Ethernet frame containing the DHCP ACK is sent (unicast) by the router to the switch. Because the switch is selflearning and previously received an Ethernet (containing the DHCP request) from Bob’s laptop, the switch knows to forward a frame addressed to 00:16:D3:23:68:8A only to the output port leading to Bob’s laptop. 包含DHCP ACK的以太網幀從路由器發(fā)送到交換機。因為交換機有自我學習的功能，所以在之前接收到從Bob發(fā)來的DHCP請求幀，交換機就知道轉發(fā)幀從接收請求幀的那個端口發(fā)送DHCP ACK 到MAC地址為00:16:D3:23:68:8A。7. Bob’s laptop receives the Ethernet frame containing the DHCP ACK, extracts the IP datagram from the Ethernet frame, extracts the UDP segment from the IP datagram, and extracts the DHCP ACK message from the UDP segment. Bob’s DHCP client then records its IP address and the IP address of its DNS server. It also installs the address of the default gateway into its IP forwarding table. Bob’s laptop will send all datagrams with destination address outside of its subnet At this point, Bob’s laptop has initialized its networking ponents and is ready to begin processing the web page fetch. Note that only the last two DHCP steps of the four presented in chapter 4 are actually necessary.When Bob types the URL for into his web browser, he begins the long chain of events that will eventually result in Google’s home page being displayed by his web browser.Bob’s web browser begins the process by creating a TCP socket that will be used to send the HTTP request to . In order to create the socket, Bob’s laptop will need to know the IP address of .We learned that the DNS protocol is used to provide this nametoIPaddress translation service.Bob的電腦接收到包含DHCP ACK的幀，一層層解封裝之后得到DHCP ACK報文。然后Bob的DHCP客戶端記錄它的IP地址和DNS服務器的IP地址。同時也將默認網關的地址記錄在它的轉發(fā)表中。Bob的電腦開始發(fā)送帶有目的地址的數據報到默認網關。此時，Bob的電腦已經開始初始化網絡部件并準備開始生成并加工網頁。注意只有最后兩個DHCP步驟在第四章中提出的四個步驟實際上是必要的。當Bob使用URL用瀏覽器打開谷歌首頁時，事件鏈最終將導致谷歌的主頁在他的網頁瀏覽器中顯示。Bob的瀏覽器開始通過創(chuàng)建一個被用來發(fā)送HTTP請求道谷歌首頁的TCP socket進程。為了創(chuàng)建套接字，Bob的電腦需要知道谷歌的IP地址。我們學習的DNS協議就是用來提供這這種名字到IP地址翻譯的服務。8. The operating system on Bob’s laptop thus creates a DNS query message, putting the string “” in the question section of the DNS message. This DNS message is then placed within a UDP segment with a destination port of 53 (DNS server). The UDP segment is then placed within an IP datagram with an IP destination address of (the address of the DNS server returned in the DHCP ACK in step 5) and a source IP address of . Bob電腦的OS創(chuàng)建一個DNS查詢消息，其中把谷歌域名放在DNS報文的請求問題部分。DNS報文被封裝在UDP數據報中，目的端口號為53.，9. Bob’s laptop then places the datagram containing the DNS query message in an Ethernet frame. This frame will be sent (addressed, at the link layer) to the gateway router in bob’s school’s network. However, even though Bob’s laptop knows the IP address of the school’s gateway router () via the DHCP ACK message in step 5 above, it doesn’t know the gateway router’s MAC addressIn order to obtain the MAC address of the gateway router, Bob’s laptop will need to use the ARP protocolBob的電腦將包含DNS請求報文的數據封裝在以太網幀頭中。該數據幀將被發(fā)送到處在學校網絡上的默認網關。然而即使Bob的電腦可以通過DHCP ，但還是無法知道其MAC地址。為了獲得默認網關的MAC地址還需要ARP協議。10. Bob’s laptop creates an ARP query message with a target IP address of (the default gateway), places the ARP message within an Ethernet frame with a broadcast destination address (FF:FF:FF:FF:FF:FF) and sends the Ethernet frame to the switch, which delivers the frame to all connected devices, including the gateway router. Bob的電腦新建一個ARP查詢消息到默認網關，并把ARP消息封裝在以太網幀進行廣播然后發(fā)送以太網幀到交換機，交換機傳幀到直連的設備，包括默認網關。11. The gateway router receives the frame containing the ARP query message on the interface to the school network, and finds that the target IP address of in the ARP message matches the IP address of its interface. The gateway router thus prepares an ARP reply, indicating that its MAC address of 00:22:6B:45:1F:1B corresponds to IP address . It places the ARP reply message in an Ethernet frame, with a destination address of 00:16:D3:23:68:8A (Bob’s laptop) and sends the frame to the switch, which delivers the frame to Bob’s laptop. 網關路由器接收包含ARP查詢消息的幀到連接學校網絡的接口。網關路由器因此準備回復ARP請求，表明它的MAC地址00:22:6B:45:1F:。網關路由器將ARP請求報文封裝在以太網幀中，其目的地址為Bob電腦的MAC地址，并且發(fā)送幀到交換機，交換機再發(fā)送在、數據幀到Bob的電腦。12. Bob’s laptop receives the frame containing the ARP reply message and extracts the MAC address of the gateway router (00:22:6B:45:1F:1B) from the ARP reply message.Bob的電腦接收包含ARP請求報文的幀并從ARP請求報文中提取網關路由器的MAC地址。13. Bob’s laptop can now (finally!) address the Ethernet frame containing the DNS query to the gateway router’s MAC address.Note that the IP datagram in this frame has an IP destination address of (the DNS server), while the frame has a destination address of 00:22:6B:45:1F:1B (the gateway router). Bob’s laptop sends this frame to the switch, which delivers the frame to the gateway router. Bob的電腦現在能發(fā)送包含DNS查詢報文的以太網幀到網關路由器的MAC地址。注意在這個幀的IP報文段中有目的IP為DNS Server的IP，目的MAC為網關路由器的MAC。Bob的電腦發(fā)送幀到交換機，然后發(fā)送到網關路由器。14. The gateway router receives the frame and extracts the IP datagram containing the DNS query. The router looks up the destination address of this datagram () and determines from its forwarding table that the datagram should be sent to the leftmost router in the Comcast network in Fi