Problem 8 (16 pt) (a) Consider the following example, a student connects her lap
ID: 3735748 • Letter: P
Question
Problem 8 (16 pt) (a) Consider the following example, a student connects her laptop to the school network using an Ethernet cable. She wants to browse www.google.com. The school network is connected to Comcast ISP A day in the life: scenario browser ONS Comcast rnetwork 68 80.0,013 school network 68.80 2.0/24 Google Googie s network 64 233.160 01 web server 64.233.169 105 ) When the laptop connects to the school network, it communicates with the D server. What addresses of what devices does the laptop learn from the D messages? (You get full credits by answering two correct addresses) [4 pts) HCP ACK (ii) How does the laptop learn the MAC address of the first-hop router in the school network? Just answer the name of the protocol that the laptop should use) [2 pts] () For the laptop to know the IP address of wwww google.com. choose the names of protocols used in any of all layers during this process, from the following list. DNS, HTTP, UDP, TCP, IP, ATM, ALOHA, Ethernet, 802.11 [6 pts] (iv) Suppose Google uses OSPF as its intra-AS routing and Comcast uses RIP. What routing protocols are running on the gateway router in Google that connects Comcast? (Hint: two routing protocols) [4 pt]Explanation / Answer
(i) A DHCP server is used to configure proper subnet mask, default gateway, DNS server informations on device and to issue and assign unique IP addresses within a network.
In small scale business or at homes, the router acts as a DHCP server. In large scale business areas, a single computer acts as a DHCP server.
Let me explain you the process that how a client/device (laptop) communicates with DHCP:
1. When a device is turned on and if it is connected to a network having a DHCP server, it sends a request called DHCPDISCOVER request to the server.
2. When the DHCP server receives this request, it holds an IP address for that device and offers it with DHCPOFFER packet.
3. Once the offer has been made for the chosen IP address, the device responds to the DHCP server with a DHCPREQUEST packet to accept it.
4. After this, the server sends an ACK used to confirm that the device has the specific address assigned to it and whether the device can use this address or not.
5. If the server decides that this device cannot have the IP address, it simply sends a NACK.
All of this happens very quickly, and you dont need to know any of the techical details. So just keep these things in mind.
(ii) ARP(Address Resolution Protocol) is the protocol which maps Ip address to the corresponding MAC Address that is recognized in a perticular local host. When you are browsing Google.com, the DNS will resolve the Name to IP. Once you have this IP, using ARP we can find the MAC address. Since, the required website is out of your network, we have to send our ARP request to Default Gateway and it is the one which actually replies with its MAC.
(iii) layer 1(network access layer)-ethernet
layer 2(internet layer)- IP, ARP
layer 3(transport layer)- TCP
layer 4(application layer) - DNS, HTTP
(iv) Since, this is a large organization, it uses Enhanced Internet Gateway Routing Protocol(EIGRP) as the Internet Gateway Protocol (IGP) using AS. Because it is multihomed (i.e., connects to two different service providers), it uses BGP to explicitly control how traffic enters and leaves the AS.