ARIEL Local Area Networking (LAN) Experiments

ARIEL Local Area Networking (LAN) Experiments

The experiments are described in chronological order, starting with very elementary ones.  Your understanding of each experiment may rely on your having completed the previous one, so if the explanation is not clear, check back to the previous experiment to get a starting point.

You can download a zip file of all five LAN experiments here. After downloading, unzip and put all the experiments in a convenient directory on your local host machine.


LAN 1. Virtual Computer – MAC Address

LAN 2. Ethernet Hub

LAN 3. Ethernet Switch

LAN 4. Virtual Local Area Networks (VLANS)

LAN 5. Spanning Tree

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LAN 1. Virtual Computer – MAC Address

The first experiment is the simplest possible.  Open GNS3 and click on the icon of the collection of four different device types in the left margin (browse all devices), select ‘installed appliances’ from the dropdown list to see several simple appliances that come pre-installed.  Click on the Virtual Computer VPCS and drag and drop it in the work area.  Click on the green “Start Triangle”.  After starting, right-click on the Virtual PC and select “Console”.  You should now have a terminal window into your Virtual PC. Try it out – see what it can do.

In the console enter the command ‘help’ to see a full list of commands.  Try ‘arp’ to see the Address Resolution Protocol cache – at this stage it should be empty.  Try ‘show ip’ which shows the information for IP (Internet Protocol) and other settings.  Scan down the list for the MAC address – this is the Medium Access Control address – in this case it is an Ethernet address (by far the most common).  Don’t worry about all the details shown.

Sample Output:

  VPCS-1>   show ip
  NAME  : VPCS-1[1]
  IP/MASK  :
  GATEWAY  : VPCS-1[1]
  DNS  :
  MAC  : 00:50:79:66:68:00
  LPORT  : 10000
  MTU  : 1500


  1. Is this a simulation of a PC or an emulation?  What is the difference?  Is the precise terminology important?
  2. How many hexadecimal digits are needed to express the MAC (Ethernet) address? 
  3. How many millions (billions?) of Ethernet addresses are possible in principle? 
  4. How many per person would that amount to if they were equally distributed to the human population of planet earth?
  5. Would that be a sufficient number of addresses for everyone? Could it stretch to the Internet of Things? 
  6. Could Ethernet addresses be used in a global addressing and routing scheme – if not, why not?

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LAN 2. Ethernet Hub

If you want to skip building the experiment manually, click on ‘File’, ‘Import portable project’ and import the GNS3 importable project ARIEL_LAN_02.gns3project, then skip ahead to “Begin here if you downloaded ARIEL_LAN_02.gns3project”

From Experiment LAN_1, drag and drop two more VPCS into the work area. Drag and drop a pre-installed Ethernet Hub into the work area. Click on the link icon, and connect each of the VPCS to a different port* on the Ethernet Hub.

* Definition of “port” In networking, the word “port” is tricky – it has different meanings in different contexts. When talking about devices, it refers to a physical input or output on the device. You can physically plug an optical or copper cable, with a suitable connector, into a compatible port. Of course, if the device is virtualised it has no physical ports, but we still refer to its virtual inputs and outputs as ports. Whether they are physical or virtual, for each port there may be several distinct interfaces – logical connection points served by that single port. Even this distinction is sometimes blurred. For example, we talk of the “loop-back port” which is more like an interface than a port.

In the context of higher layer protocols the word “port” is interpreted differently. In the Transport Layer, 16-bit port numbers are used to address separate applications. A Transport Layer segment within an IP Packet (possibly within an Ethernet Frame) is directed to an interface by the IP address, and then, within the host served by that interface, it is directed to an application by the port number. The application connects to the port by a software “socket”.

In a way, the two meanings are related – in both cases a port is the thing on which the transmission comes in and goes out.

Tip: if you want your layout to look neat, turn on ‘Snap to Grid’ and ‘Show Grid’ under the menu ‘Edit’, ‘Preferences’. If you want to, you can take a screenshot by clicking on the camera icon in the top bar.

Click on the green triangle to start the emulation. Then click on the ‘>_’ icon to start up a console for all three VPCS. There is no console for the Ethernet Hub because it is a “dumb” device. An Ethernet hub is sometimes described as a “bus in a box”, where “bus” refers to a length of (originally coaxial) cable into which end devices may tap.

Begin here if you downloaded: ARIEL_LAN_02.gns3project.

Check that all three ARP tables are empty. Then, without being concerned about the details or the reason, assign an IPv4 address to each VPCS. The command for VPCS-1 is:


Then for VPCS-2 and VPCS-3 use “ip", and “ip". Note what happens if you deliberately make a mistake and try to assign to VPCS-3 as well as VPCS-1.

Start a packet capture on “Link 1” – the link from VPCS-1 to the Ethernet Hub – right click on the link and select ‘Start capture’. Click on ‘OK’ for the default settings.

Using the IP addresses, we can run the program “ping” to generate some traffic. In the console for VPCS-1 enter the command ‘ping’ to cause VPCS-1 to “ping” VPCS-2.

The first two packets captured are ARP – Address Resolution Protocol – packets. This allows the devices in the network to learn which IP address is associated with which MAC address. The first packet is a broadcast, saying:

“Who has Tell”

The Ethernet hub sends this broadcast out on every port except the one it came in on. At this stage, only VPCS-2 knows that its IP address is It receives the broadcast packet and sends a reply to the originator of the request, giving its MAC address. The corresponding MAC and IP addresses are then stored temporarily in the ARP table in the interface of the PC (why temporarily?).

After the ARP procedure, you should see five pairs of packets – each pair comprising an “echo request” and an “echo reply”. The time between sending the echo request and getting the echo reply is a measurement of the Round Trip Time (RTT).

Now enter ‘arp’ commands again, and carefully check the information in each ARP table. Can you explain what you see? If the ARP tables are empty, try the ping again and check the ARP tables quickly, before the entries in the tables expire. The tables are, in fact, caches.

Now quickly repeat the ping from VPCS-1 to VPCS-2. Do you see any ARP packets? Can you explain? If you wait a couple of minutes and ping again, do you see any ARP packets. Can you explain?

Wait a couple of minutes for all entries in all ARP caches to expire, then ping from VPCS-2 to VPCS-3. Watch for packets being captured on the link from VPCS-1 to the Ethernet Hub. Would you expect to see any packets on this link, since it has no direct connection to either VPCS-2 or VPCS-3. Explain what you see.

Are you confident you could explain what packets you expect to be captured on each link, and what entries will be in each ARP table, as each node pings any other node?

Sample Output:

VPCS-1> ip
Checking for duplicate address...
PC1 :

VPCS-1> ping

84 bytes from icmp_seq=1 ttl=64 time=1.035 ms
84 bytes from icmp_seq=2 ttl=64 time=1.102 ms
84 bytes from icmp_seq=3 ttl=64 time=0.948 ms
84 bytes from icmp_seq=4 ttl=64 time=1.012 ms
84 bytes from icmp_seq=5 ttl=64 time=0.867 ms

VPCS-1> arp

00:50:79:66:68:01 expires in 63 seconds 

Wireshark Packet Capture Files:

Packets that were captured on Link 1 while VPCS-1 pings VPCS-2: ARIEL_LAN_02_Capture_01.pcapng

Packets that were captured on Link 1 while VPCS-2 pings VPCS-3: ARIEL_LAN_02_Capture_02.pcapng


  1. After you have completed the experiment and shut down GNS3, you can still examine the packet capture files using Wireshark on your own machine (independently of GNS3).
  2. Assuming you have Wireshark installed on your machine, double click on the file ARIEL_LAN_02_Capture_02.pcapng and you can review the details of the packets captured on the Link from VPCS-1 to the Hub while VCPS-2 pinged VPCS_3.
  3. How are Ethernet addresses used to achieve point to point communication over a broadcast medium? How can you broadcast information to every workstation connected to a single Ethernet Hub?
  4. For research (Googling): what is the structure of an Ethernet address, and how are they assigned? Are they globally unique?
  5. Is it fair to call an Ethernet hub a “dumb” device? Is it a useful device? What if every one of your devices were located in a different city, and you had to pay for all traffic on every link?
  6. For research (Googling): What is a Frame Check Sequence?
  7. Each of the four numbers in the IPv4 addresses (192.168.1.X) can range from 0 through 255 (in principle). How many bits make up an IPv4 address, and how many millions (billions) of IPv4 addresses are possible? Are there enough IPv4 addresses to give one to every person on earth? Are there enough for the Internet of Things?
  8. Why measure Round Trip Time, wouldn’t it be better to measure the time in each direction, especially if the forward and return paths are different, as they might be in a big network?
  9. The “echo request” and “echo reply” packet begins with two Ethernet addresses, and then moves on to the two IP addresses. Examine the packet capture log to find the addresses.
  10. Each IP address is followed by a mask (or a qualifier like /24, for example). What does this qualifier mean? Hint: Google the term “CIDR”

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LAN 3. Ethernet Switch

From Experiment 2, stop the emulation, then right click on the Ethernet Hub and delete it. Then replace the Hub with an Ethernet Switch. Reconnect the three VPCS. Start the emulation again and spin up consoles for all devices. The Switch gets a console because it is a “Smart” device.

Repeat the procedure for experiment 2 – confirm or configure IP addresses, then ping from host to host and examine the ARP tables.

Explain the changes you observe from Experiment 2 to Experiment 3.

Wireshark Packet Capture Files:

Packets that were captured on Link 1 while VPCS-1 pings VPCS-2: ARIEL_LAN_03_Capture_01.pcapng

Packets that were captured on Link 1 while VPCS-2 pings VPCS-3: ARIEL_LAN_03_Capture_02.pcapng


  1. Does the Ethernet Switch make better use of the available transmission resources than the Hub did?
  2. Does it make sense that the Ethernet Switch has a console, but the hub does not?
  3. The Console for the Ethernet Switch does not offer many options. Instead, try the “Configure” menu by right-clicking on the switch icon in the workspace.
  4. Can you explain how the Ethernet Switch “learns” about the MAC addresses of the devices connected to it, and how it uses that information? Would you expect the behavior of a switch to change a little over time, after it is first switched on?

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LAN 4. Virtual LANS

To begin, here is a riddle: What is the difference between “invisible” and “virtual?  Answer:  if something is invisible you CANNOT see it, even though it IS there – if something is virtual you CAN see it, even though it is NOT there. 


A Virtual Local Area Network, or VLAN, is a virtual network within a physical network.  It operates as a separate LAN, but it exists only in the form of tags that are added to Ethernet frame headers.    The devices in the network can “see” the VLAN even though the VLAN is not there, physically.



IEEE 802.1Q, often referred to as Dot1q, is the networking standard that supports VLANs on an Ethernet network. The standard defines a system of VLAN tagging for Ethernet frames and the accompanying procedures to be used by bridges and switches in handling such frames.

This diagram from shows how the Dot1q header, or VLAN tag, is slotted in after the Source MAC address.

In this experiment, you will configure one physical LAN with two VLANs, and then demonstrate what is meant by: “You can see the VLAN even though it is not there, physically.”

Start a new project with one Ethernet Switch and four VPCS terminals.  BEFORE making any connections, right click on the Ethernet Switch icon and select the ‘Configure’ option.   The ports on the Ethernet Switch can be modified by double clicking on the port then entering the required options.  IMPORTANT: click “Add” after you have made modifications for each port.  If you have trouble with a port, “Delete” that port and “Add” a new one with the same number, configured as you prefer.  Finally, if you like, you can “Delete” all the remaining ports you don’t need.  Your configuration should end up like the Figure above.   Note that port 0 is designated as dot1q and VLAN 1 – this implies that VLAN1 is the “Native VLAN”.

Spin up 5 VPCs, configured with IPv4 addresses:

  VPCS-0>   ip

Here is a screenshot of my workspace, with added notes for clarity:


  1. Can you predict who can ping whom? Check it out experimentally.
  2. Discuss: “you can see the VLAN even though it is not there (physically)”.

  3. For research: is it possible to use more than one VLAN tag on a single frame?
  4. Examine the Packet Capture and see whether you can identify the VLAN tags (I could not find them – I suspect that the switch emulation is not 100% faithful).

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LAN 5. Spanning Tree

What might happen if the switching topology became a little more complex.  Suppose Electrical Engineering (denoted “E”), and Mechanical Engineering (denoted “M”), and Chemical Engineering (denoted “C”) all had their own Ethernet Segment, and one Ethernet Switch each.  Then suppose each department wanted to connect to the other two.  We would end up with a triangle of three Ethernet Switches denoted E, M, and C.  If the link from E to M is down, we would want the E-to-M traffic to go from E via C to M, rather than just getting dropped.  But, that degree of flexibility implies the possibility of a routing loop coming into existence.  If all three links are working, and traffic from E to M is routed via C, perhaps also the traffic from C to M is routed via E.  So where does E-to-M traffic flow.  First it goes via C on the way to M.  At C it is destined for M, so it is sent via E – back to where it started.  In principle, the process continues indefinitely.

Most Switches and Bridges are too smart to allow infinite loops to develop.  They are often referred to as “Learning-Switches” and “Learning-Bridges”.   They implement a Spanning Tree Protocol, with the aim of ensuring that, at any time, there is just one path from any Ethernet segment to any other segment.  The existence of one and only one path is the definition of a “tree network”; the “spanning” term just means that all nodes are included – no-one is unreachable.  They achieve the tree topology be effectively banning some paths. 

The disadvantage of a tree is that any single link failure results in the tree being cut into two disconnected parts.  As links go down or up over time, the paths may have to change, but the ideal is to avoid routing loops, even transient ones.

The built-in Ethernet Switch in GNS3 does not implement STP, therefore we will move on to Open vSwitch (see for our next experiment.  Even Open vSwitch does not have Spanning Tree Protocol, or the more modern Rapid Spanning Tree Protocol, enabled by default – we will have to enable one or the other through a Command Line Interface (CLI). 


Later we will discover by experiment that Open vSwitch is an SDN (Software-Defined Networking) device – it can be controlled comprehensively from an external controller, instead of relying on a CLI.  Check out the Open Daylight web site ( for a preview.  By the way, both Open Daylight and Open vSwitch come under the Linux Foundation (  While you are there, check out the free MOOCs available from the Linux Foundation.

We begin with three Open vSwitches and three VPCs.  Connect each VPC (1 through 3) to the corresponding Open vSwitch, but do not use interface eth0 on any of the switches, because that interface is reserved for management.  Connect Open vSwitch-1 to Open vSwitch-2, and Open vSwitch-2 to Open vSwitch-3, but do not complete the loop from 3 to 1.

Start the emulation, and spin up consoles for all devices.  Configure the usual – or unusual J – IPv4 addresses for the PCs, and save the configuration.  Now try pinging.  Hopefully you will find that every PC can ping every other PC.


Check to see if the Round Trip Times between 1 and 3 tend to be greater than the times between 1 and 2 – why would that be so?  Let’s fix that by adding a link from Open vSwitch-1 directly to Open vSwitch-3 – what could go wrong?  Does the additional link speed up the round trip?  What happened?

In the console for each Open vSwitch enter the following three commands to enable STP – Spanning Tree Protocol:

ovs-vsctl set Bridge br0 stp_enable=true

ovs-vsctl set Bridge br0 other_config:stp-priority=0x7800

ovs-vsctl set Port eth0 other_config:stp-path-cost=10



  1. Does enabling STP fix the problem immediately?  Does it fix the problem after a while?  Hint: try pinging immediately after enabling STP.

  2. For research: what is the difference between RSTP and STP? What other advanced switch protocols and algorithms are there?
  3. Discuss what you have learne from all the ARIEL LAN experiments – do you agree you have a good basis for learning more about switching?

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