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SFWRTECH 3IT3 Lab #1 Layer 3 and 4 Protocols
12 min.

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

# VM

Default gateway :

Windows1 IP : Windows2 IP : CentOS01 IP : (with httpd) CentOS02 IP :

# Step 8

ipconfig /all

# Step 9

netsh interface ipv4 set dns "Ethernet" static primary
netsh interface ipv4 add dns "Ethernet"
ipconfig /all

Differences between dynamic and manual IP config:

  • DHCP enabled in dynamic and disabled in manual
  • DHCP server in dynamic is but disappeared in manual

# Step 12

Show ARP cache

arp -a


Physical Address of : 08-00-27-c8-13-4b

Interface: --- 0x3
internet Address | Physical Address | Type         | Purpose           52-54-00-12-35-00  dynamic        Default Gateway           08-00-27-75-7c-58  dynamic        DHCP Server           08-00-27-2b-a4-3e  dynamic        Neighbouring VM         ff-ff-ff-ff-ff-ff  static         Directed Broadcast         01-00-5e-00-00-16  static        01-00-5e-00-00-fb  static        01-00-5e-00-00-fc  static    ff-ff-ff-ff-ff-ff  static         Flooded Broadcast were appeared after browsing a website.

Wireshark OUI Lookup Tool:

OUI for 08-00-27 : PCS Computer Systems GmbH

# Step 13

Using Windows1 , delete all entries in ARP cache arp -d

arp -d
arp -a
arp -a

A new entry 08-00-27-2b-a4-3e was created in ARP cache after ping .

There is no ARP entry for default gateway because Windows2 is in the same subnet that Windows1 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 from 08-00-27-2b-a4-3e to 08-00-27-11-22-33 and run arp -a on Windows1 , the ARP entry of Windows2 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 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 , Windows1 receives 4 Request timed out on running ping .

# Step 19

su -
yum -y install nc httpd
yum -y install nc

# Step 20

Create index.html file

<html><body>Hello World</body></html>

Save to /var/www/html/index.html

# Step 21

Run ifconfig or ip addr on CentOS01, find device enp0s3 with inet


firewall-cmd --zone=public --add-port=80/tcp --permanent
firewall-cmd --reload

on CentOS01

Run ifconfig or ip addr on CentOS02, find device enp0s3 with inet

# 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 by browsing

# Step 24

Run nc -ul 1234 on CentOS01 and nc -u 1234 on CentOS02

Then type abc on CentOS02 and press Enter. I receive "no route to host".

Then release port number 1234 using udp from firewall of CentOS01

firewall-cmd --zone=public --add-port=1234/udp --permanent
firewall-cmd --reload

# Step 25

If I first try to send a message from CentOS01 , nothing happens, because UDP protocal is connectionless. The server CentOS01 is just listening to port 1234 but it does not have connection to the client CentOS02 because the client CentOS02 does not need to build connection with the server CentOS01 very firstly before sending out messages.

# Step 26

Run nc -l 5678 on CentOS01 and nc 5678 on CentOS02 .

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 so CentOS02 knows the connection is aborted.

# Step 27

Release port number 5678 using tcp from firewall of CentOS01

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 , and the client CentOS02 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 , the server CentOS01 is also down because the client CentOS02 sends out a FIN flag and expects to end the connection. The server CentOS01 sends a FIN flag again with a ACK flag for confirming, and the client CentOS02 confirms that. TCP protocal only allows point to point transport due to one connection.

# Step 29

Run route -n on CentOS01 to identify the default gateway.

Run cat /etc/resolv.conf on CentOS01 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.


  1. A network with hosts that have a NIC binded with multiple IP addresses
  2. 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?

Answer: Subnet Mask is so it is actually a Class C => Network Address . 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)?


  1. Ask DHCP server to get a valid IP address.
  2. Ping all possible IP addresses to see which is not in use.
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