Subnetting Demystified: Concepts, Importance, and a Real-World Example

#networking #cybersecurity #tutorial #linux

Prerequisites

The goal of this article is to give you a clear understanding of what subnetting is, its importance, and a real-world example of subnetting in action. To fully grasp the concepts in this article, you need a good understanding of what an IP address is, its structure, subnet masks, and CIDR notation. All of these topics are covered in this article: [Decoding IP Addresses: Mastering Subnet Mask and CIDR Notation].

How Subnetting Works

Subnetting is the method used to divide large networks into smaller, more manageable networks called subnetworks or subnets.
A network is simply a group of computers that can communicate with each other. In technical terms, it is a group of hosts within the same IP range. As the size of a network grows, communication becomes increasingly difficult due to heavy traffic loads. To address this issue, large networks can be divided into smaller, more manageable subnetworks (subnets) using a process called subnetting. Subnetting helps reduce congestion by limiting broadcast traffic to smaller groups of hosts.

Flat Network

A flat network is a network without subdivisions (subnets), where all devices share the same broadcast domain. This can lead to issues like increased broadcast traffic, scalability problems, and security risks, which subnetting aims to address.

Example of Subnetting

Suppose you have a Class C IP address 192.168.1.0/24 that needs to be divided into 4 subnets. The total number of possible IPs is 256, ranging from 192.168.1.0 to 192.168.1.255.

Note: Only 254 IP addresses are available for use because the Network address (192.168.1.0) and the Broadcast address (192.168.1.255) are reserved and cannot be assigned to devices. The Network address is the first IP address in the range, and the Broadcast address is the last.

The subnet can be divided as follow:

Subnet 1: 192.168.1.0/26. This provides a total of 64 IPs from range 192.168.1.0 to 192.168.1.63
Subnet 2: 192.168.1.64/26. This provides a total of 64 IPs from range 192.168.1.64 to 192.168.1.127
Subnet 3: 192.168.1.128/26. This provides a total of 64 IPs from range 192.168.1.128 to 192.168.1.191
Subnet 4: 192.168.1.192/26. This provides a total of 64 IPs from range 192.168.1.192 to 192.168.1.255.

Importance of Subnetting

Some of the importance of dividing a network into subnetworks include:

Efficient Use of IP Addresses: Subnetting allows the administrator to allocate only the necessary number of IPs required for that subnet, preventing waste.
Improved Security: Subnetting reduces security risks by limiting access between subnets. Unlike a flat network, which broadcasts packets and resources to every single host on the network.
Enhanced Performance: Each subnet runs faster because of the reduced broadcasting traffic burden. For example, it is easier to broadcast a packet to 10 hosts in a subnetwork than to 200 hosts in an undivided, flat network.
Simplifies Network Management: By dividing networks into subnetworks, the workflow on a topology becomes clearer, making it easier to manage, detect problems, and reduce downtime. When one network goes down, the others can still function properly.
Privacy: Subnetting limits visibility between subnets by isolating sensitive traffic from the general network. For example, the HR department does not need to see the data that travels within the Admin department.

A Real-World Application of Subnetting

Your company has been assigned the IP network: 192.168.1.0/24. As the Network Admin, you are tasked with dividing this network to serve four different departments in your company:

Admin (50 hosts)
HR (25 hosts)
Staff (30 hosts)
Guest (14 hosts)

Each subnet must accommodate the required number of hosts. The formula for calculating how many hosts are available to a network is:

Hosts Available = 2^h - 2

Where:

h is the number of host bits in the IP address.
2 is subtracted because the network address and the broadcast address are reserved and cannot be used.

Finding the Number of Host Bits (h)

Let's use the Admin subnet as an example. Remember, the Admin subnet needs 50 hosts.

To find h for this subnet, find the smallest power of 2 that can accommodate the required number of hosts (50) in the subnet using this formula:

2^h ≥ Required Hosts + 2

**Number of host bits(h)** => 2h ≥ 50 + 2

After solving, h will equal 6 (approximately 5.7, which rounds up to 6).

We can solve for the total number of hosts that will be available to the subnet:

**Hosts Available** => 26 - 2 = 62

The subnet mask for this subnet will be: 32 - 6, where 32 is the number of bits in an IP address and 6 is the number of host bits (h). This will give us a subnet mask of /26.

Allocating the Admin Subnet

From the example above, we can now set the Admin Subnet as follows:

- Department: Admin
- Number of host bits(h): 6
- Hosts Needed: 50
- Hosts Available: 62
- Subnet mask: `/26` or `255.255.255.192`
- Network Address: `192.168.1.0`
- Broadcast Address: `192.168.1.63`
- Usable IP Range: `192.168.1.1` to `192.168.1.62`

Using this example, allocate the subnet for the HR, Staff, and Guest subnets. If you get stuck anywhere, feel free to reach out to me or leave a comment.