Suppose you walk into a room, connect to Ethernet, and want to download a Web pa

Question

Suppose you walk into a room, connect to Ethernet, and want to download a Web page at Assume there is nothing in the DNS or browser caches when you power on your PC. In order to send a Web request, your computer first uses DHCP to obtain an IP address. What IP datagram should your computer send out first? Please indicate the source and destination address of this IP datagram, and how it is forwarded in the Ethernet. Your computer needs to send a DNS request to get the IP address of the web server How can your computer find the IP address and the MAC address of the local DNS server? Once your computer has the IP address of the Web page, it will send out HTTP request to retrieve the page. Describe how the HTTP request on the application layer is encapsulated and forwarded on the way to the Web server. When the Web server received the HTTP request, it will send back the Web page via IITTP response messages. Describe how these HTTP response messages are encapsulated and forwarded on the way back to your computer.

Explanation / Answer

First, I turn on a PC and broadcast a package to request for IP address. When this package is broadcast
on routers it’ll get an IP address and return to my PC. Then I broadcast an ARP package. Router will
update its ARP cache after receiving this ARP package, along with saving my PC’s IP address and MAC
address. Then I type in web address in my browser. The browser will find IP address from DNS via DNS
protocols. After that, my PC has already got the IP address of the typing web address. Then it’ll
construct a TCP connection via transport layer and web server with port 80. Then PC will construct a
TCP package with server address, port 80 and the request webpage and deliver it to network layer.
Network layer will add IP head to the package with PC’s IP address as source address and webpage’s
IP address as destination address and transmit to link layer. Then router will send such package to
website via router table and website will return PC’s request and finally dispose TCP connection.

Your computer first uses DHCP to obtain an IP address. You computer first creates a special IP
datagram destined to 255.255.255.255 in the DHCP server discovery step, and puts it in a Ethernet frame and broadcast it in
the Ethernet. Then following the steps in the DHCP protocol, you computer is able to get an IP address with a given lease time.
A DHCP server on the Ethernet also gives your computer a list of IP addresses of first-hop routers, the subnet mask of the
subnet where your computer resides, and the addresses of local DNS servers (if they exist)

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