Course section: SFWRTECH 3IT3:Fundamentals of Networking - SFWRTECH_3IT3_poonh_2205
SFWRTECH 3IT3 – Fundamentals of Networking
Lab #1: Layer 3 and 4 Protocols
Following & Answers Record
Default gateway : 10.0.2.1
Windows1 IP : 10.0.2.4 Windows2 IP : 10.0.2.5 CentOS01 IP : 10.0.2.8 (with httpd) CentOS02 IP : 10.0.2.9
# Step 8
# Step 9
netsh interface ipv4 set dns "Ethernet" static 220.127.116.11 primary netsh interface ipv4 add dns "Ethernet" 192.168.1.1 ipconfig /all
Differences between dynamic and manual IP config:
- DHCP enabled in dynamic and disabled in manual
- DHCP server in dynamic is 10.0.2.3 but disappeared in manual
# Step 12
Show ARP cache
Physical Address of 10.0.2.4 : 08-00-27-c8-13-4b Interface: 10.0.2.4 --- 0x3 internet Address | Physical Address | Type | Purpose 10.0.2.1 52-54-00-12-35-00 dynamic Default Gateway 10.0.2.3 08-00-27-75-7c-58 dynamic DHCP Server 10.0.2.5 08-00-27-2b-a4-3e dynamic Neighbouring VM 10.0.2.255 ff-ff-ff-ff-ff-ff static Directed Broadcast 18.104.22.168 01-00-5e-00-00-16 static 22.214.171.124 01-00-5e-00-00-fb static 126.96.36.199 01-00-5e-00-00-fc static 255.255.255.255 ff-ff-ff-ff-ff-ff static Flooded Broadcast 188.8.131.52 were appeared after browsing a website. Wireshark OUI Lookup Tool: https://www.wireshark.org/tools/oui-lookup.html OUI for 08-00-27 : PCS Computer Systems GmbH
# Step 13
Windows1 10.0.2.4 ,
delete all entries in ARP cache
arp -d arp -a ping 10.0.2.5 arp -a
A new entry
10.0.2.5 08-00-27-2b-a4-3e was created in ARP cache after
ping 10.0.2.5 .
There is no ARP entry for default gateway because
Windows2 10.0.2.5 is in the same subnet that
Windows1 10.0.2.4 in, so data can be sent directly to neighbours and not to the gateway to handle further.
I can use
ipconfig command to get the ARP entry for default gateway.
# Step 14
After change MAC of
Windows2 10.0.2.5 from
08-00-27-11-22-33 and run
arp -a on
Windows1 10.0.2.4 , the ARP entry of
Windows2 10.0.2.5 is dynamically changed to
08-00-27-11-22-33 . (yes, if I switch Windows1 quickly enough, the MAC is not changed yet)
I ping to
Windows2 10.0.2.5 and receive 4
Request timed out after MAC changed.
# Step 15
After I select “Turn on Windows Defender Firewall” under the
“Public network settings” on
Windows2 10.0.2.5 ,
Windows1 10.0.2.4 receives 4
Request timed out on running
ping 10.0.2.5 .
# Step 19
su - yum -y install nc httpd yum -y install nc
# Step 20
# Step 21
ip addr on CentOS01, find device
firewall-cmd --zone=public --add-port=80/tcp --permanent firewall-cmd --reload
ip addr on CentOS02, find device
# Step 22
Start Apache web server
systemctl enable httpd.service systemctl start httpd.service
# Step 23
I can see the "Hello World" page through
CentOS02 10.0.2.9 by browsing
# Step 24
nc -ul 1234 on
CentOS01 10.0.2.8 and
nc 10.0.2.8 -u 1234 on
CentOS02 10.0.2.9 and press Enter. I receive "no route to host".
Then release port number
udp from firewall of
firewall-cmd --zone=public --add-port=1234/udp --permanent firewall-cmd --reload
# Step 25
If I first try to send a message from
CentOS01 10.0.2.8 , nothing happens, because UDP protocal is connectionless. The server
CentOS01 10.0.2.8 is just listening to
port 1234 but it does not have connection to the client
CentOS02 10.0.2.9 because the client
CentOS02 10.0.2.9 does not need to build connection with the server
CentOS01 10.0.2.8 very firstly before sending out messages.
# Step 26
nc -l 5678 on
CentOS01 10.0.2.8 and
nc 10.0.2.8 5678 on
CentOS02 10.0.2.9 .
This returns "no route to host" again because
port 5678 is not released from the firewall. A TCP
RST flag is sent back to
CentOS02 10.0.2.9 so
CentOS02 10.0.2.9 knows the connection is aborted.
# Step 27
Release port number
tcp from firewall of
firewall-cmd --zone=public --add-port=5678/tcp --permanent firewall-cmd --reload
# Step 28
I first try to send a message from the server
CentOS01 10.0.2.8 , and the client
CentOS02 10.0.2.9 gets the message because both sides must have connection built under TCP protocal before any messages sent.
When I press
CTRL + C on the client
CentOS02 10.0.2.9 , the server
CentOS01 10.0.2.8 is also down because the client
CentOS02 10.0.2.9 sends out a
FIN flag and expects to end the connection. The server
CentOS01 10.0.2.8 sends a
FIN flag again with a
ACK flag for confirming, and the client
CentOS02 10.0.2.9 confirms that. TCP protocal only allows point to point transport due to one connection.
# Step 29
route -n on
CentOS01 10.0.2.8 to identify the default gateway.
cat /etc/resolv.conf on
CentOS01 10.0.2.8 to check DNS config.
# Post-Lab Questions
An ARP cache may contain multiple entries with the same MAC address, even though the IP addresses listed are different. This commonly occurs in some networks. Describe the configuration of network devices that could result in this behaviour.
- A network with hosts that have a NIC binded with multiple IP addresses
- Web server binded with IP addresses, so that the website can be surfed through multiple IP addresses
In classful IP addressing, what classful network would your Windows and CentOS IP addresses normally belong to? What “class” do they actually belong to, and why? Write the network address for the VMs in CIDR notation. What is the maximum number of hosts that can be configured in this subnet?
Subnet Mask is
255.255.255.0 so it is actually a
Class C => Network Address
10.0.2.0/24 . Number of possible hosts:
2 ^ 8 - 2 = 254 .
How could a computer detect an IP address conflict in the network when starting up, before it finishes loading its own IP network configuration? In other words, how could a computer determine whether an IP address is already in use on the network before it attempts to configure its network interface using the same address (which would then result in a conflict)?
- Ask DHCP server to get a valid IP address.
- Ping all possible IP addresses to see which is not in use.