Experiment 3: IP Sub-networks and Static Router Configuration


  1. Setup an isolated network with 2 LANs. Determine network parameters for hosts in sub-networks.

  2. Manually configure the network parameters for a client workstation, a router connecting the two LANs and a host for a web server.

  3. Observe the traffic generated by the traceroute program to verify connectivity.



Before performing this experiment, review the following topics from class notes: Internet layers, IP addressing, IP routing, Ethernet network concepts, ICMP protocols.

The network topology is shown below. Note that there is no external connection to Internet and that allows us to use any arbitrary addresses without interfering with the exterior world. IP addresses in the figure are just illustrative values: actual values for the experiment will depend on the group number and the organization ID assigned by the lab instructor. Note that the router has two IP addresses, one for each LAN.

Isolated network topology diagram


  1. Ask the lab instructor to be assigned a group number (n), a virtual LAN network ID, and the port numbers to connect to the 3 virtual machines for this experiment. In this manual, we’ll assume that the data provided is the following:

    • Virtual LAN ID:

    • Port for client workstation (client1) in LAN1: 11000

    • Port for router: 11001

    • VNC password and account information for client1, router and server1

    • Port for web server in LAN2 (server2): 11002

  2. Before you type anything on the computer, you must sub-divide the assigned LAN in two sub-networks, one for LAN1 and one for LAN2. Calculate the corresponding masks, which will be needed for the router configuration file and assign an IP address to all network interfaces (2 in the router and one on each station). Normally the router for a network is assigned the first few host addresses for the network. For example, if LAN1 was assigned for the network ID, you should configure the corresponding interface of your router at, as shown in the diagram. It is recommended to draw your own diagram with the IP address for each interface to guide you during the configuration steps.

  3. Launch 3 UltraVNC instances, one for each of the 3 stations. The addresses to connect should look similar to the following:

  4. Begin by configuring the router first. This router firmware is configured by default to be used in embedded devices without a keyboard and a screen. As the router is not intended to run regular user applications, only basic utilities are available to reduce the memory requirements in embedded devices. Usually, the only way to access a command line is by using a remote computer connected to a network interface (using telnet or ssh), or using a serial terminal program. To simplify access for this experiment, we have enabled a virtual console, accessed through the VNC protocol. Press enter to start a shell in the console. A warning message will inform that there is no root password, which is not safe. For this experiment, the warning may be ignored as the networks are isolated. Run ip -f inet addr show (or abbreviated ip -4 a show) to see IPv4 configured interfaces (assigned ip addressed may be different):

    # ip -4 a show
    1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN qlen 1000
        inet scope host lo
           valid_lft forever preferred_lft forever
    3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP qlen 1000
        inet brd scope global eth1
           valid_lft forever preferred_lft forever
    4: br-lan: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000
        inet brd scope global br-lan
           valid_lft forever preferred_lft forever

    The router has 4 network interfaces defined by default:

    • lo: is the loopback network interface, configured to This is a special address that always points to the localhost (the router in this case).

    • eth0: connected to LAN1, part of a bridge (br-lan)

    • eth1: connected to LAN2

    • br-lan: is a virtual bridge that has eth0 as one of its ports. Since eth0 is included in the bridge, we’ll directly configure the br-lan device and ignore eth0. You may list the interfaces (there is only one by default) connected to this bridge by running the following command:

      # brctl show br-lan

    Initially, the router interfaces will be configured to some generic IP addresses. We will change these to the addresses assigned to the group. In OpenWrt, top-level configuration files are located in /etc/config. the basic network parameters can be set by editing the network file. You may use vi or nano to edit text files in the router. If you are not familiar with any of those, use nano, which is aimed at beginners:

    # nano /etc/config/network

    You only need to update the lan (br-lan interface) and wan (eth1) sections to reflect the network IP addresses and netmasks assigned to your group. In our example, the lan section is assigned with a mask of, and the wan section to with a similar mask:

    config device
            option name 'br-lan'
            option type 'bridge'
            list ports 'eth0'
    config interface 'lan'
            option device 'br-lan'
            option proto 'static'
            option ipaddr ''
            option netmask ''
    config interface 'wan'
            option device 'eth1'
            option proto 'static'
            option ipaddr ''
            option netmask ''

    The new router configuration can be activated by running the following command:

    # /sbin/reload_config

    If no errors occur, verify that the interfaces have been assigned the correct IP addresses:

    # ip -4 address show

    Include the output of this in your report (for example using a screen capture). Note: OpenWrt also includes the ifconfig utility in the default setup, so you may also use that one if you prefer. For the purpose of this experiment, the router should be now ready.

  5. Configure the client. Log-in into client1 and open a terminal. The ip command can be used to manually configure the network interface. Run ip a show (without the -4)to see all interfaces:

    client1:~$ ip a show
    1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
        link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
        inet scope host lo
           valid_lft forever preferred_lft forever
    2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
        link/ether 52:54:00:e0:36:eb brd ff:ff:ff:ff:ff:ff

    Only the loopback interface is currently configured. The Ethernet interface is named enp1s0 (check the name you get, it may be different). There is no active connection to other computers in the network. We will use a static configuration. First, we must elevate the privileges to make system changes. This can be achieved using the sudo command. We will assign the IP address and add the route to the table directly from the command line using ip command. This is a good approach to change the network configuration on the fly, but keep in mind that the configuration is lost after a reboot. The first command configures the interface (enp1s0) using the default broadcast address but keeps it disabled, the second command enables the interface:

    client1:~$ sudo ip addr add <address/bits> broadcast + dev <interface>
    client1:~$ sudo ip link set <interface> up

    Example for our sample assigned addresses:

    client1:~$ sudo ip addr add broadcast + dev enp1s0

    Check the interfaces are properly configured using the ip -4 a show command. If correct, you should be able to ping the router now:

    client1:~$ ping -c 3
    PING ( 56(84) bytes of data.
    64 bytes from icmp_seq=1 ttl=64 time=0.107 ms
    64 bytes from icmp_seq=2 ttl=64 time=0.279 ms
    64 bytes from icmp_seq=3 ttl=64 time=0.432 ms
    --- ping statistics ---
    3 packets transmitted, 3 received, 0% packet loss, time 2094ms
    rtt min/avg/max/mdev = 0.107/0.272/0.432/0.134 ms

    Print the client routing table:

    client1:~$ ip route

    As we are doing things manually, there should be no default route. Add the default route to point to the router, to forward any packets not in LAN1 to the router:

    client1:~$ sudo ip route add default via <router IP>

    Run again ip route to check the result. Include this routing table in your report. You can try pinging the other router IP address. It should work.

  6. Configure the web server. Repeat the same steps you used for the client, using the IP addresses in LAN2 instead of LAN1. Try pinging the client. It should be reachable.

  7. Launch wireshark on the client and start capturing traffic on the Ethernet interface. Use the following command to trace the route from the client to the webserver:

    client1:~$ traceroute <server2 IP>

    After the route is printed, stop capturing packets in Wireshark. Examine the captured packets due to the traceroute command. Explain how this sequence of messages work, noting the TTL field in the outgoing packets and the type of ICMP messages received in your report.

  8. You may now close wireshark, open a web browser in client1 and connect to the web server by using the following URL: http://<server2 IP>/. For our example, Verify that the information given by the webserver is correct, and include a screen capture in your report.

  9. After the experiment is finished, close all VNC clients.

Report preparation and questions

  1. Prepare a formal report summarizing this experiment in pdf format and submit it to the lab instructor. Report writing rules:

    • One report per group

    • All students are responsible for the contents of the report, but one student in the group must coordinate, write and submit the report for the experiment.

    • Clearly state in the report cover the name of all students in the group and indicate who prepared the report

  • Include in your report a diagram of the network topology used in the experiment. Show network ID for each segment, IP address for each interface and routing table in each host.

  • Include outputs indicated in boldface in some of the steps.

  • Use the packets captured with wireshark to explain how the

    traceroute program determines the route to a host.

  • Write comments conclusions about this experiment.