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TCP/IP Networking Exercises

The purpose of this exercise is to get some practice with using ping, netstat, tcpdump, traceroute, arp, and route.

Before we start we need to make sure that we have tcpdump and traceroute installed on our PC.

To do this, run:

sudo apt-get install traceroute tcpdump

If the packages are already installed, apt-get will simply say so. If they are not installed, it will download and install the two packages (and any supporting packages required).

Check your network configuration

Check it with:

$ ifconfig eth0

Do you see an IP address on your network card? It should look like this:

eth0      Link encap:Ethernet  HWaddr 52:54:8e:12:66:49
          inet addr:10.10.0.xx  Bcast:  Mask:

where xx is your machine's IP address.

DO NOT DO THE FOLLOWING - it is just for information.

If the eth0 network card does not have a 10.10.0.xx IP address, then it would be configured as follows:

$ sudo ifconfig eth0 10.10.0.xx/24
$ sudo route add default gw

You were asked not to do the above because you are logging in using ssh via the network interface, so you will end up breaking your access to your machine.

You machine has other interfaces. Use what you've just done above to look at the other interfaces eth1 and lo.


Look at your routing table:

$ netstat –rn

What do you notice? Is the default gateway configured? How do you know? Review the presentation if you are not sure. What is your default gateway? On what network interface is your default gateway valid for?

Here's another way to look at your routing table:

$ ip route


Let's ping the default gateway:

$ ping

(Stop it with CTRL+C)

Let's ping something outside, on the Internet. For example,

$ ping

Do you get an answer ?

If not, check:

  • That you have a gateway configured
  • That in the file /etc/resolv.conf there is an entry for “nameserver”
  • Do you notice anything about the response time? How far away is

Verify is configured as your default gateway:

$ ip route

Now, remove your default gateway:

$ sudo ip route delete default via

Check that it's gone

$ ip route

How can you be sure that the default gateway is no longer configured? Now, try to ping the local WWW machine.

$ ping

Now let's ping a machine outside our network (

$ ping

The ip address of is

$ ping

What do you observe?

What is the consequence of removing the default gateway?

Re-establish the default gateway:

$ sudo ip route add default via

Check that the default gateway is enabled again by pinging

$ ping


Traceroute to

$ traceroute

Try again, this time with the -n option:

$ traceroute -n

Observe the difference with and without the '-n' option. Do you know what it is?


Run tcpdump on your system:

$ sudo tcpdump -n -i eth0 icmp

(Note the use of the icmp keyword to limit viewing ICMP traffic)

Ask the instructor(s) or your neighbour to ping your machine, and look at your screen.

master/cnd/tcp-ip-lab.txt · Last modified: 2016/03/24 09:56 by philip