DT008/3
Communications Systems 1
Laboratory 1
Objective
This laboratory is an introduction to communications networks. You will become familiar with
some basic operations that you will perform frequently during the course.
Check that the Windows Firewall is turned off. From the Start menu, select Control Panel, System
& Security, Windows Firewall.
Download the Results sheet from Webcourses and record your results on the sheet. Upload your
results sheet on Webcourses before the next laboratory.
Lab Procedure:
Exercise 1: Collect Network Settings:
From the Start menu, click on the Run command, and then enter the parameter cmd;
alternatively, go to Accessories and start Command Prompt .
In the window that opens, type the command ipconfig (then press enter) and take note of the
IPv4 address that is displayed in the window after Ethernet adapter Local Area Connection.
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The IP address uniquely identifies a device to make it reachable from any location using the
Internet protocol.
The IP address is described as a sequence of 4 decimal values, representing 4 bytes.
Type the command ipconfig /all and note the Physical Address that is displayed in the window
after Ethernet adapter Local Area Connection.
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The Physical address uniquely identifies a device, but can be used only to reach it from within a
Local Area Network, using the Ethernet protocol.
The Physical address is described as a sequence of 6 pairs of hexadecimal values, representing
6 bytes.
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Exercise 2: Test Connectivity:
PING (Packet Internet Groper) uses a protocol of the TCP/IP suite, ICMP (Internet Control
Message Protocol), to test the connectivity between 2 computers. When one host ''pings'' another
host on a network, an ''ICMP Echo request'' packet is sent to the destination. If the destination
host receives the request, it will reply with an ''ICMP Echo reply'' packet.
In the command prompt window, type ping address where address can be the IP address of
another PC in the room.
Note how many replies do you get (each reply corresponds to a single request)?
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Note that some parameters of this test can be adjusted.
Use the following command: Ping –n 1 <address>
What is the effect of this parameter (how many replies)?
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Use the following command: Ping –l 512 <address>
What is the effect of this parameter (how many bytes of data)?
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Exercise 3: Conversions
We frequently will need to convert data between the different formats:
Convert decimal => binary
Convert your IP address into a binary sequence of 4 bytes:
Example:
8.16.4.1 <=> 00001000.00010000.00000100.00000001
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Convert binary => decimal
Convert the following binary sequence of 4 bytes into an IP address of 4 decimals
11000000.10101000.00101101.01111001
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Convert Hexadecimal => Decimal
Convert your Physical Address into a decimal sequence, converting Hexadeimal value by pairs.
Example:
A1:3C <=> 161:60
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Convert binary => hexadecimal
Convert 11001110.01011010 into hexadecimal
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Convert decimal => hexadecimal
Convert 147.252.127.255 into hexadecimal.
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Exercise 4: Wireshark
Wireshark is a network protocol analyser. It captures messages received by or transmitted from
the computer on which it runs. It then analyses the messages to interpret and display their
content.
You can start the Wireshark program from the desktop or the Start menu.
The Wireshark graphical user interface has 4 major elements.
Display Filter
Packet Listing
Packet Header
Details
Packet Contents
- The packet listing window displays a one-line summary for each packet captured. The listed
information usually includes: the packet number (assigned by Wireshark), the time at which the
packet was captured, the packet’s source and destination addresses, the protocol type, and
protocol-specific information contained in the packet. The packet listing can be sorted according
to any of these categories by clicking on a column name.
- The packet header details window provides details about the packet selected (highlighted) in
the packet listing window. (To select a packet in the packet listing window, place the cursor over
the packet’s one-line summary in the packet listing window and click with the left mouse button.).
These details include information about the Ethernet frame and IP datagram that contains this
packet. The amount of details displayed can be expanded or minimized by clicking on the + sign
on the left of the line in the packet details window.
- The packet-contents window displays the entire contents of the captured frame, in both ASCII
and hexadecimal format.
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- The packet display filter window, is used to enter information in order to filter the packets
displayed in the packet-listing window (and hence the packet-header and packet-contents
windows), based on this information (e.g. you may only want to see the packets where your IP
address appears as source or destination).
The File menu allows you to save captured packet data or open a file containing previously
captured packet data.
The Capture menu allows you to begin packet capture.
If you select Options in this menu, you can use most of the default values in this window.
The Interface pull down menu at the top of the Capture Options window shows the
network interfaces (i.e., the physical connections) that the computer has to the network.
Select an interface that is being used to send and receive packets.
Capture Filter can be used to specify conditions to filter the packets to be captured.
Note that the syntax for capture filters and display filters is not the same.
After selecting the network interface (or using the default interface chosen by Wireshark), click
Start. Packet capture will now begin.
Once the capture is started, perform from the command prompt window a ping towards another
ip address. Once you see the completion of the test in the command prompt window (replies
received), stop the capture in Wireshark.
Display Filters
(1) You can build a Display filter by following the instructions:
Click on the button marked “Expression”. In the left panel of the “Filter Expression” window, scroll
down to the value IP. Expand IP.
Scroll down to the expression: ip.src and then highlight it. In the Relation panel, select ==, and in
the value window enter your IP address.
Click on apply and describe the result (what is the source IP address of the messages that are
left?).
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(2) Clear the display filter.
Enter in the filter window the condition: icmp
Click on apply and describe the result (what is the protocol type of the messages that are
left).
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(3) Clear the display filter.
Create a display filter that will keep messages containing your IP address as source or
destination.
Write down the expression you entered.
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