Case Study Exploration Network Fundamentals
Naam: Pieter-Jan Liekens
Student: S5058883
Vak: Cisco Network Fundamentals
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17 /12/2009
klas: 1 Ti7
Case Study
Exploration
Series 9
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CCNA Exploration: Network Fundamentals (Guided Case
Study)
Learning Objectives
• Get used to the Packet Tracer 4.1 tool.
• L2 and L3 addressing scheme.
Background
In order to send/receive packet through a data computer network, 2 types of
addresses are necessary: Layer 2 and Layer 3 network addresses. (also known as L2
and L3 addresses)
While L2 addresses have a local scope, which allows another device to have the same
L2 address in a different network segment, L3 addresses have a global scope. A L3
address assigned to a device must be unique all over the internet during the time this
device is connected to the network.
In order to achieve data delivery, L2 addresses are (statically or dynamically)
mapped to L3 addresses.
The address resolution protocol (arp) is a protocol used by the Internet Protocol (IP)
[RFC826], specifically IPv4, to dynamically map IP network addresses (L3 addresses)
to the hardware addresses (L2 addresses) used by a data link protocol. The protocol
operates below the network layer as a part of the interface between the OSI network
and OSI link layer. It is used when IPv4 is used over Ethernet.
The Internet Control Message Protocol (ICMP) is used by send error and control
messages between devices. ICMP is a TCP/IP Network Layer protocol, first defined in
RFC 792, September, 1981. ICMP message types were later expanded in RFC 1700.
An extremely useful application called ping uses ICMP as the main protocol.
Task 1: Scenario
In this task, you will be asked to address and ping the devices on the same network.
After the addressing is done, you will learn more about the arp protocol.
Task 1: Address the PCs
On your work PC, run the Packet Tracer 4.1 and use the configuration that you
made for the production department . It contains 3 PCs connected to a switch. Assign
IP addresses and subnet masks to the PCs. Don’t worry about the DNS and default
gateway for now. The PCs IP Addresses must match the table below:
PC1:
IP Address: first useable ip address
Subnet Mask: Fill in the right subnetmask
PC2:
IP Address: second useable ip address
Subnet Mask: Fill in the right subnetmask
PC3:
IP Address: last useable ip address
Subnet Mask: Fill in the right subnetmask
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Note: Since the addresses are being specified by you, the Static option must be
chosen.
The topology is shown below:
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Task 1: Verifying the addresses on the PCs
Once you have all the addresses assign to the PCs, check their addresses using the
command ipconfig /all. The output of this command shows the L2 and the L3 address
assign to the PC.
While the L3 address was assign by you, the L2 address was acquired from the
network installed on the PC. The output of that command when issued on PC1 should
look like this:
PC>ipconfig /all
Physical Address............: 00D0.BC00.5910
IP Address......................: 10.10.10.11
Subnet Mask.....................: 255.255.255.0
Default Gateway.................: 0.0.0.0
DNS Servers.....................: 0.0.0.0
PC>
On the output above the Physical Address line shows the L2 address of PC1
The IP Address line shows the L3 address of PC1, which was assigned by you.
Note: Since Packet Tracer is a simulation program, the PC’s Physical Addresses might
be different on your system than the ones showed here.
Question 1: List the L3 and L2 addresses of the PCs and paste your results in the
appropriate box.
For PC3:
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Task 1: Ensuring Connectivity
ping operation is straight forward. The source computer sends an ICMP echo request
to the destination. The destination responds with an echo reply. If there is a break
between the source and destination, a router may respond with an ICMP message
that host unknown or destination network unknown.
Ensure the PCs are able to reach each other by using the ping command. From each
PC, ping the other 2 to ensure connectivity:
The output of that command when issued on PC1 should look like this:
PC>ping 10.10.10.12
Pinging 10.10.10.12 with 32 bytes of data:
Reply from 10.10.10.12: bytes=32 time=157ms TTL=120
Reply from 10.10.10.12: bytes=32 time=75ms TTL=120
Reply from 10.10.10.12: bytes=32 time=94ms TTL=120
Reply from 10.10.10.12: bytes=32 time=84ms TTL=120
Ping statistics for 10.10.10.12:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 75ms, Maximum = 157ms, Average = 102ms
Paste your results in the appropriate box.
From PC1
From PC1 to PC2
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From PC1 to PC3
From PC2
From PC2 to PC1
From PC2 to PC3
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Case Study Exploration Network Fundamentals
From PC3
From PC3 to PC1
From PC3 to PC2
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Task 2: Connecting to a Server
Task 2: Scenario
On Task 2 will learn more about clients and servers by placing a request from a client
to a server. This task will use the HTTP as the illustrative protocol spoken between
the client and the server.
The topology is shown below:
Task 2: Preparing the Devices
In computer networks a very common structure is a Client-Server structure. In this
structure, the device which requests information is called a client and the server
which provides the information is called Server.
Run the Packet Tracer 4.1 program and load the ccna_discovery_cs_task2.pkt file
into it.
The topology is also shown below:
On this topology the PC1 will act as a client and the Server1 will act as a server. In
order to have connectivity between PC1 and Server1, assign L3 addresses to PC1 and
to Server1 according to the table below:
PC1: 192.168.10.11
Subnet Mask: 255.255.255.0
Server1: 192.168.10.25
Subnet Mask: 255.255.255.0
Note: Again, chose the Static as address type and don’t worry about the DNS and
default gateway information for now.
Once the addresses were assigned, issue a ping command from PC1 to Server 1.
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Task 2: Ensuring connectivity
The http protocol will be protocol spoken between the client and the server during
this task. Since HTTP is a layer 7 protocol, ensure connectivity on the lower layers
before start the upper layers communication.
Which command can be used to ensure connectivity on the lower layers for example
between PC1 and the server?
Fill in the answer!!
Answer: Ping
In the next box, place the result of that command.
If the command was not succesful, troubleshoot your installation.
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Task 2: Requesting a HTTP page from the Server
Since the ping command from PC1 to Server1 was successful, it is safe to state that
the lower layers connectivity was achieved. The network is now ready to support the
upper layer protocols traffic.
Open a web browser on PC1, type the Server’s 1 ip address on it (192.168.10.25)
and press enter. You should see a web page stored at Server1.
PC1, acting as a HTTP client, sends a HTTP request to Server1 asking for a web page
located on that server. Since HTTP protocol uses the TCP protocol on port 80, this
request is sent as TCP packet from PC1 to the Server1’s port 80.
Server1 is running HTTP server software and thus, is ready to respond HTTP
requests. After Server1 receive the HTTP packet from PC1, it interprets the request
and answers properly. PC1 receives a TCP packet from Server1 containing a web
page and shows it on the web browser window. PC1 acted as client and requested
information. Server1 acted as server and provided a response regarding the PC1’s
request.
Task 2: The ARP Protocol
In order to send packets, the source and the destination devices must learn their L2
addresses and map it to the respective L3 address. Since this is an Ethernet
environment, the L2 address is the MAC address.
The ARP protocol provides a way to dynamically map L2 to L3 addresses by querying
all the devices on the segment about their L2 addresses. Suppose PC1 is trying to
send data to PC3, below is a summary of the ARP protocol operation:
1. PC1 needs to know the MAC address of the PC3 before it can send any data. PC1
sends a broadcast packet out on the network. This packet has the question “Who has
the IP address 192.168.10.13?” (192.168.10.13 is the IP address of PC3). This
packet is known as ARP request.
2. PC2 receives the question but, since PC2’s IP address is 192.168.10.12 (and not
192.168.10.13), PC2 does not answer the question. The reason why PC2 receives the
packet is the packet was a broadcast.
3. PC3 also receives the packet. PC3 checks the question the answers it because
192.168.10.13 is PC3’s IP address.
4. PC3 sends an UNICAST packet back to PC1 saying: “Hey, this PC3. I have
192.168.10.13 and my MAC address is: 000D.BD62.D826. This answer is known as
ARP reply
5. Now PC1 has PC3’s MAC address and PC3 has PC1’s MAC address and the data
communication between PC1 and PC3 can take place.
Notice that:
- Even though the process was initiate by PC1, at the end PC3 also knows PC1 MAC
address. This is a feature of the ARP protocol. It assumes since PC1 needed PC3’s
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MAC address, chances are PC3 will send data to PC1 shortly and thus, PC3 also stores
PC1’s MAC address. This keep PC3 to start a new ARP query to learn PC1’s MAC
address.
- PC2 has no information about PC1 or PC3 MAC addresses because the process did
not include PC2.
The ARP protocol keeps a table on the device’s memory called ARP table. This table
maps L3 addresses to L2 address of the devices to which the local device sends data.
Task 2: The ARP table
The arp -a command can be issued on the PCs in order to list their ARP table.
Check the ARP table on PC1, PC2 and PC3 and answer the questions below.
At this point, what is the content of PC1, PC2 and PC3 ARP table? Fill in the answer!!
Answer: PC1:
Internet Address
Physical Address
192.168.10.25
0006.2abe.7989
PC2, PC3: nothing
Type
dynamic
From PC1, ping to PC3. The ping should be successful.
Check the ARP table on PC1, PC2 and PC3 again and answer the questions below:
List the content of PC1 and PC3’s ARP table and place them in the appropriate box.
The content of the ARP from PC1
The content of the ARP from PC3
Answer: PC3 has obtained the Mac address belonging to the PC1-IP. PC1 has also
added PC3 in its own list.
What is the content of PC2’s ARP table? Why? Nothing, because there has not been
any Arp-request from or to PC2.
The content of the ARP from PC2
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Answer: As you can see, the Arp-table from PC2 is empty. It does not contain any
IP’s because no requests have been sent from or to PC2...
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Task 3: Getting out of the Local Network
Since the L2 addresses are unique within the same segment, within the same local
network (local scope) why is the L3 addresses necessary? Fill in the answer!!
Answer: In remote networks, sending packets requires knowledge of the physical
address (L3)
So far all the packets were sent/received to/from devices within the same local
network. The necessity of an upper layer address, a L3 address, rises when data
must be sent to remote networks.
Run Packet Tracer 4.1 and load the ccna_discovery_cs_task3.pkt file. The topology is
shown below:
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When a packet must be sent out of the local network, an intermediate device must be
used. Such device must be connected to the local network and to the remote
network. This intermediate device is called router or gateway. (The term Gateway is
more often used from the PCs viewpoint)
Observation: Usually, the intermediate device is not directly connected to the remote
network. In those cases, it must have a path (or a route) through another
intermediate device to reach the remote network. The process of learning paths and
choosing the best one is called routing.
From PC1, ping the PC3 computer. The ping should be successful. Place the output of
your command in the next box. Place also the output of the arp –a command in the
box.
Ping from PC1 to PC3
What is the address listed on PC1’s ARP table? Fill in the answer!!
Answer: 000d.bd62.d826
Internet Address
10.10.10.13
Physical Address
000d.bd62.d826
Type
dynamic
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Now, still from PC1, ping Server2 (20.20.20.25). Place the output of your result in
the next box.
Ping from PC1 to Server2
Ping from PC1 to PC3
Why PC1 is able to ping PC3 but it is not able to ping Server2? Fill in the answer!!
Answer: PC1 does not know the physical address of Server 2. Server2 is in a
different network. Not in the same, local network.
A Default Gateway is an intermediate device used when the destination device lies in
a remote network. The local device concludes the destination device is not on the
same network and thus the data must be forwarded to a third intermediate device,
an endpoint device. This device is called Default Gateway.
PC1 has no default gateway information configured on it and thus is not able to reach
any device which is not on its local network.
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On the topology shown above, Router1 (10.10.10.1) is the default gateway for the
PC1’s network. When Router receives a packet from PC1, it checks the destination IP
address of the packet. Since the destination IP address is Server2’s IP address and
not Router1’s IP address, Router1 understands it must route the packet. Router1
search its routing table looking for a matching route to the destination 20.20.20.25.
Router1 finds a route to the network 20.20.20.0/24 via fa0/0 and forward the packet
through that interface.
Open the PC1’s configuration tab and let Router1 (10.10.10.1) be PC1’s default
gateway.
Now, issue the same ping again from PC1 to Server2. Place the output of your result
in the next box.
The ping still fails. Why? Fill in the answer.
Answer: The server hasn’t got a default gateway set.
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Set the default gateway to be used by Server2.
Remember: A default gateway is the device used by the local network devices to
reach networks out of the local network scope.
What is the IP address to be used as a default gateway on Server2?
Answer: 20.20.20.1
After you have the correct default Gateway configured on Server2, issue the ping
command from PC1 to Server2 again. Place the output of your result in the next box.
The ping is now successful.
Once you have established the lower layers and routing connectivity, access the web
page stored at Server2 via HTTP protocol by opening a web browser at PC1 and
typing Server2 IP Address on it.
The web page stored on Server2 is now shown.
Task 3: The ARP Table When Sending Packets to Remote Metworks
Check PC1’s ARP Table again. Place the output of this command in the next box.
The ping to Server2 was successful but why the entry for Server2 (20.20.20.25) is
not listed on the ARP Table? What is the address listed on the PC1’s ARP Table? Fill in
the answer!!
Answer: The default gateway-adress – In this case the Mac address of Router2
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