Cisco’s Skills for All platform provides several free courses in networking and more. For this lab, we’re getting started with Cisco Packet Tracer.

Cisco Packet Tracer is a network simulation tool developed by Cisco Systems. Packet Tracer allows users to design, configure, and troubleshoot network topologies and imitate modern computer networks, without the need for physical hardware.

Watch this video for an overview of Cisco Packet Tracer, how to set it up, and the main features: Cisco Packet Tracer | Everything You Need to Know

Set up a free account at Cisco Networking Academy (https://www.netacad.com/). Then sign up for the free Getting Started with Cisco Packet Tracer Course (https://skillsforall.com/course/getting-started-cisco-packet-tracer?courseLang=en-US) to access Packet Tracer.

Work through this exercise, including all of the sections in Modules 1 and 2 below.

Module 1: Download and Use Cisco Packet Tracer

• 1.0. Install Cisco Packet Tracer
• 1.1. The Cisco Packet Tracer Interface

Module 2: Create a Cisco Packet Tracer Network

• 2.0. Packet Tracer File Types
• 2.1. Build a Home Network
• 2.2. Summary of Course

How does this home network compare to your own?

Suppose you are designing a network for a small startup and you want to obtain a block of public Class C addresses. In this lab, we will shift gears a “bit” to practice selecting and subnetting a Class C network..

Class C addresses are used for small to medium-sized networks and have a default subnet mask of 255.255.255.0. This means the first 24 bits of the IP address are used to identify the network, and the remaining 8 bits are used to identify specific devices, or hosts, on that network.

CIDR notation is a method used to represent the network portion of an IP address and the number of significant bits used for the network. It allows for more flexible allocation of IP addresses compared to the traditional classful addressing scheme.

In CIDR notation, a Class C address is represented with a /24 prefix, indicating the number of network bits in the network portion of the address. For example, an IP address of 192.168.1.0 with a subnet mask of 255.255.255.0 can be represented in CIDR notation as 192.168.1.0/24. The last, or fourth, octet will hold the host addresses of each machine on the network.

Source:

“Network Fundamentals – Internet Protocol and IP Addressing | Information Security | University of Houston-Clear Lake.” Available: https://www.uhcl.edu/information-security/tips-best-practices/ipaddressing.

Step 01: Select a Class C Public Network Address

Select a random Class C public IP address of your choice. You can select your address manually, following the definition of a Class C public IP address. Alternatively, you can generate an IP address with this Python script: Class-C-Address.ipynb

What is your chosen public Class C (or CIDR /24) IP address? Why is this a Class C or CIDR /24 address?

Step 02: Translate Your Address and the CIDR Prefix to Binary

Now, let’s translate the four octets into binary. You can use an online converter to help if you wish. Here is Rapid Table’s converter: Convert Decimal to Binary.

Make sure and pad the lefthand bits with zeros as needed. So your output should look something like this sample translation from another IP address: 11000110 11110000 11101001 11011001

Now translate your CIDR prefix to binary. For example, if the CIDR prefix was /25, the first 25 bits refer to the network portion of the address: 11111111 11111111 11111111 10000000

What is your network address translated to binary? What is the CIDR prefix of your network in binary (also known as your “subnet mask”). Show your work.

Step 03: Mask Your IP Address Using the Bit AND Operator

In this step, you will mask your IP address to determine the network address (or “subnet”).

Remember that computers see everything as binary (1,0). Therefore, we need to understand data masking, which is based on the binary operation ‘AND’.  Here are the rules:

• 0 & 0 = 0
• 0 & 1 = 0
• 1 & 0 = 0
• 1 & 1 = 1

Performing this operation bitwise between two binary numbers results in a new binary number with 1’s in every space where both original numbers had 1’s, and 0’s everywhere else.

Here is an example with a Class B address 172.16.0.1 with a subnet mask of 255.255.192.0.

1. Translating to binary:

The IP address 172.16.0.1 in binary is 10101100.00010000.00000000.00000001, and the subnet mask 255.255.192.0 in binary is 11111111.11111111.11000000.00000000.

2. Performing the AND operation:

• • IP Address:                       10101100 00010000 00000000 00000001
  • Subnet Mask:                   11111111 11111111 11000000 00000000
  • Network Address:           10101100 00010000 00000000 00000000

3. Converting the network address back to IPv4:

Converting the result back to the network address, we get 172.16.0.0. Therefore, the network address for the Class B IP address 172.16.0.1 with a subnet mask of 255.255.192.0 is 172.16.0.0.

What is your network address in binary and IPv4 format? Show your work with the binary AND operation as with the Class B example.

Step 04: Identify Characteristics of Your Network

It’s important to know special addresses on your network. Here are some examples:

• The first and last addresses for any network are reserved for the network itself and the broadcast address.
• The broadcast address would be one in which all host bits were set to 1.
• The last host address is the one before the broadcast address (by one bit).
• The next network would start with the next bit after the broadcast address.

What is the first host address in your network? What is the broadcast address of your network, in binary and in IPv4 format? What is your last host address? What would be the next adjacent network available? Show your work and reasoning.

Step 5: Describe How to Subnet Your Network

Now that you’ve created a public Class C network, let’s return to your own home network ipconfig output. Answer the following questions:

• Is your IP address a public or private address?
• What class address is your address?
• How many IP addresses are usable in your network? We know that the total number of bits in all IP addresses is 32 bits. The number of host bits is equal to 32 minus the number of network bits. The first address and last address are reserved, as mentioned before. So, we can use the formula 2^{number of host bits – 2}

You can use this site to check your work:

• “1.5. Understanding CIDR Subnet Mask Notation.” Available: https://doc.m0n0.ch/quickstartpc/intro-CIDR.html.

Next, consider why you would want to subnet your network after reading the following article:

• “Subnetting Your Home Network: A How-To Guide,” Jan. 10, 2024. Available: https://todaysautomatedhome.com/home-networking/subnetting-your-home-network-a-how-to-guide/.

Think through this example, in which we create two subnets in a Class C network with the address 192.168.1.x, with these steps:

• Determine the Subnet Mask: For two subnets, you need to borrow 1 bit to create the subnets. This results in a subnet mask of /25, which provides 2 subnets.
• Allocate IP Ranges: With a /25 subnet mask, each subnet will have 126 usable IP addresses. You can allocate the IP ranges for the two subnets as follows:
  1. 192. Subnet 1: 192.168.1.0/25 (192.168.1.1 to 192.168.1.126)
     193. Subnet 2: 192.168.1.128/25 (192.168.1.129 to 192.168.1.254)
• Configure Subnet Masks: In the subnetted network, each of the two subnets will have a subnet mask of 255.255.255.128, which is equivalent to /25 in CIDR notation.

Answer the questions in this step. Then, create a design that subnets your existing network into three subnetworks. First, define the purpose of each network, and then use subnet masking to split your existing network into three ranges.  

You can check your work with this site:

• toddlammle, “Subnetting a Class C network address,” TechRepublic, May 24, 2001. Available: https://www.techrepublic.com/article/subnetting-a-class-c-network-address/.

Subnetting takes practice!  The key here is to be able to explain the process that leads to your answers.

Additional Hints:

• You can use the Practical Networking exercise generator to check your work in this lab: https://subnetipv4.com/
• The Practical Networking videos on subnetting provide a great tutorial: Harmoush, “Subnetting Mastery – Learn to Subnet like a Pro!,” Practical Networking .net, Aug. 03, 2018. Available: https://www.practicalnetworking.net/stand-alone/subnetting-mastery/.