Lab 4-mike

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Algonquin College *

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CST8109

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Computer Science

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Jun 8, 2024

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docx

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Uploaded by SargentComputerWombat39

Lab 4: Capture/Analyse Remote Communication Process What you will do: • Using the skills and knowledge acquired in lab 02 and 03, you will build, configure, and test a Wired (Ethernet ) network consisting of fours nodes : two end devices (laptop) and two intermediary devices ( Linksys router ). • Connect to a Web server on a non-default application port. • Capture and analyse the remote communication process where both sender and receiver are on different network segments. • Reset a Linksys router to factory settings • Implement basic router configuration: Connect to router’s management web page (blue screen) o Configure a static IP on Internet interface (Linksys router) o Modify the router’s IP address o Disable Network Address Translation (NAT) o Allow anonymous Internet requests Things that you will need to know or learn: • Everything that you learned in lab 01, 02 and 03 you will need to complete this lab. • Identify and understand the different layers of addressing necessary to a successful communication. • Understand the remote communication process. • The general purpose and format of an ARP message . • Writing simple Wireshark expressions to filter frames • Understand the information provided in the Wireshark Details Pane for the purpose of extracting addressing information as well as being able to map protocols to their OSI or TCP/IP network model layers. • Connecting to a web server via a non-default application port. • Determining your network adapter’s MAC address . • Determining your default gateway’s IP and MAC address . What you need to submit and when: • Complete the in-lab part of the lab including instructor signoffs before the end of your lab period (refer to the instructions below). This part is to be completed with a partner . • Complete the “ Lab 4 Post-lab ” on Brightspace before the due time. This part is to be completed individually. Required Equipment: • Equipment requirements per team: Network cables: two straight-through and one crossover o Two Linksys routers o Wireshark installed and working on both laptops (done in Lab 01) o Lab 03 and 04 documents downloaded to your laptop o Webserver.exe downloaded to your laptop o Two laptops Marks: 10 References and Resources: • Lab 01, 02 and 03 • Cisco Chapter 3 Layer 3 and 2 Addresses Here are some useful rules to remember about addresses:

• A message’s layer 3 (e.g., IP address) and layer 4 port number (e.g., application port) values do not change as the message moves from one network to another (there are exceptions, but we will not look at these until later in the semester). The destination address value corresponds to the IP address of the device the message is ultimately intended for. o The source address value corresponds to the original message sender’s IP. • The message’s layer 2 (e.g., MAC address ) address values change as the message moves from one network to another . Here are the Layer 2 address values as the frame leaves the sender’s device. When sending a message to a local device the frame’s destination MAC corresponds to the local device’s MAC address .  The frame’s source MAC corresponds to the sending device’s MAC address. (Host MAC) When sending a message to a remote device  The frame’s destination MAC corresponds to your default gateway’s MAC address.  The source MAC corresponds to the sending devices’ MAC address. Frames are the PDUs we place on the physical media . They are responsible for carrying our messages from one device to another within the boundaries of the same network segment. If the device we wish to communicate with is on another network segment, then, the frame will be addressed and delivered to the default gateway . It is the default gateway (i.e. router) that performs the complex work of moving the message across the network. Remember that a frame has no life beyond the network segment on which it was created! The router discards the original frame and encapsulates a message in a new frame when it needs to move the message to another network. Address Resolution Protocol (ARP) The ARP protocol operates on Wi-Fi and Ethernet networks and provides the mechanism for obtaining the layer 2 addresses necessary to move frames from one device to another within the same network segment. In short, the purpose of the ARP protocol is to resolve IPv4 addresses to MAC addresses. Here is a brief and simplified description of how ARP works. • When sending a message to a local device o An ARP request seeking to obtain the local devices MAC address is broadcast to all devices on the network segment as the sender  The ARP request is of the form: • Who has a.b.c.d? Tell w.x.y.z  The ARP response is of the form: • w.x.y.z is at aa:bb:cc:dd:ee:ff o The learned MAC address is used to direct the frame to its destination device. o The learned MAC address is cached in the sending device’s local ARP memory. • When sending a message to a remote device: Remember that you direct all remote communications to your router. You let it do the hard work! o An ARP request seeking to obtain your default gateway’s MAC address is broadcast to all devices on the network segment o The learned MAC address is used to direct the frame to your default gateway device. The default gateway does all the hard work involved in internetwork communications. o The learned MAC address is cached in the sending device’s local ARP memory. Note that an ARP broadcast message does not exist/live beyond the network segment it was created on. That is a router will STOP the spread of an ARP message to other networks! Imagine the traffic that would exist if routers allowed ARPs to spread to all networks! The arp –a command allows you to display the ARP table and hence determine the MAC address of devices your device has communicated with ( locally ) . An ARP table ONLY contains MAC entries of other locally connected devices. You will never see or know the MAC address of remote device – the only MAC you need to know to communicate with remote devices is the default

gateway’s MAC address . Note that the entries in an ARP table have a time limit; they will be deleted from memory after a configurable amount of time! The figure below shows the output of running ipconfig and arp –a on a device having IP address 192.168.15.106. By examining the output below we can tell that default gateway’s MAC address for 192.168.15.106 is: 0c-47-3d-a1-88-82 . We can deduct this information by first determining the Default Gateway address from the ipconfig output. We then take this IPv4 address and attempt to find a matching entry in the arp –a output. As there is an entry in the arp –a output for 192.168.15.1, its MAC address is the value that appears under the Physical Address column! From the output we cannot tell what the MAC address for 192.168.15.106 is. In this case we would need to run ipconfig/all to obtain the MAC address for 192.168.15.106. Note that the output below shows multiple interfaces, but as we have learned in class, the interface that provides connectivity with the outside world in this specific configuration is the Ethernet adapter. Task 0: Preparations 0.1 Find a partner to work with. You must work in teams of exactly two per team. 0.2 Confirm you have downloaded the following from BS “Labs - > Lab 04” to your computer: 0.2.1 “Lab 04 – In-Lab Activies.pdf” (this document)

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