What is Subnet Mask in Computer Networking

A subnet mask is an important part of computer networking. It helps divide an IP address into two parts: the network part and the host part. This process is called subnetting.  Subnetting allows computers to communicate more efficiently within smaller networks. It also connects these networks to a larger gateway.

Using a subnet mask has several benefits. It can improve network performance and reduce congestion. For example, if you have an IP address of 192.168.1.1 and a subnet mask of 255.255.255.0, devices can easily identify their network segment. This makes connectivity and management much easier

This article will teach what is subnet mask in networking, how subnetting works, what are the needs of subnetting and its importance with an easy explanation. By the end of it, you will be proficient with the subnet mask and subnetting.

As a part of our Network security training, where we learn about various components and technologies of networking this article will delve deep into subnet mask and it's working.

What is a Subnet Mask?

A subnet mask is a 32-bit number used in networking to separate an IP address into the network and host portions. It plays a crucial role in subnetting, allowing networks to be divided into smaller, manageable sub-networks.

When devices communicate, the default gateway directs traffic between these subnets and external networks.

Subnet Mask example:  If a computer with an IP address of 192.168.1.10 uses a subnet mask of 255.255.255.0, it can identify its local network and determine if it needs to send data through the gateway to reach other networks.

The process that allows us to divide a large IP network into smaller, manageable segments is called subnetting.

What is Subnetting?

Subnetting is the process of dividing a larger IP network into smaller, manageable segments known as subnets. This technique enhances network efficiency by reducing congestion and improving performance.

Each subnet has its unique subnet mask and range of IP addresses, allowing devices within the subnet to communicate directly without routing through a gateway.

Subnetting example:  A company might use subnetting to create separate networks for different departments, ensuring better organization and security. By isolating traffic, subnetting also minimizes broadcast traffic, leading to faster communication within the network.

Subnet Mask and IP Address Classes

IP Address Classes categorize IPv4 addresses into five classes: A, B, C, D, and E. Each class serves different purposes based on the size and type of network. Subnet Masks are used in conjunction with IP addresses to determine the network and host portions of an address.

A subnet mask helps segment a network into smaller subnets, improving organization and efficiency. For example, a subnet mask of 255.255.255.0 (or /24) indicates that the first three octets represent the network portion, while the last octet identifies individual devices within that network.

Subnet Mask Table

How does Subnetting Work in Networking?

Subnetting is a process which can be understood in 6 steps:

Step 1- Assign an IP Address: When a device, such as a computer or router, connects to a network, it is assigned a unique IP address. In IPv4, this address is a 32-bit number represented as four octets (e.g., 192.168.1.1).

Step 2- Understand the Structure: An IP address consists of two parts: the network portion, which identifies the specific network, and the host portion, which identifies the individual device within that network.

Step 3- Use a Subnet Mask: A subnet mask is also a 32-bit number that helps determine which part of the IP address is the network and which part is the host. For example, a subnet mask of 255.255.255.0 indicates that the first three octets represent the network (192.168.1) and the last octet represents the host (1).

Step 4- Calculate Subnets: To create subnets, you borrow bits from the host portion of the IP address to form additional network addresses. For instance, if you need four subnets from a Class C address (default mask 255.255.255.0), you can change it to 255.255.255.192, allowing for four subnets with 62 usable host addresses each.

Step 5- Identify Network and Broadcast Addresses: Each subnet will have a unique network address (the first address in the range) and a broadcast address (the last address in the range). These addresses are essential for routing traffic within and between subnets.

Step 6- Implement Routing: Devices within each subnet can communicate directly with each other without routing through a gateway, enhancing speed and reducing overall network traffic.

By understanding and applying these steps, subnetting allows for efficient management of IP addresses while improving communication within networks.

Master Your Networking Skills!Prepare CCNA With our CCNA Practice Exam Course!Explore course

Why is a Subnet Mask Used in Computer Networks?

The subnet mask purpose is to differentiate between the network and host portions of an IP address. Here are some examples showing why a subnet is required:

1. Network Organization: Subnetting allows large networks to be divided into smaller, manageable subnets. This organization makes it easier to allocate IP addresses and enhances overall network management.

2. Improved Network Performance: By reducing broadcast traffic and network congestion, subnetting helps networks operate more efficiently. Smaller subnets mean less traffic on each segment, which can lead to faster communication.

3. Enhanced Security: Subnetting increases security by isolating different parts of the network. For example, a company can create a separate subnet for sensitive data, restricting access to authorized personnel only.

4. Multi-Vendor Integration: Organizations often use different vendors for various network components. Subnetting allows for better management and maintenance of these diverse segments, ensuring seamless integration.

5. Remote Access Management: Subnetting enables organizations to allocate specific IP ranges for remote users, allowing them to access the network securely while maintaining control over their connectivity.

6. Efficient IP Address Utilization: By dividing a large address space into subnets, organizations can make better use of their IP addresses, preventing wastage and ensuring that every device has a unique identifier.

As you can see, a subnet mask is vital for efficient network design and management, providing benefits such as improved performance, enhanced security, and better organization of IP addresses.

How to Find Subnet Mask From IP Address?

Many students face difficulties with subnet mask calculations. Although there are many subnet mask calculators available online they are useable for students as students must know how to find subnet mask by themselves.

In this section, we will understand the steps to find subnet masks and subnets with the help of an example.

Problem Statement: Let's say you have an IP address of 192.168.1.10 and you want to create a subnet mask that allows for 30 hosts in your subnet.

Step 1: Determine the Number of Hosts

First, calculate how many bits you need for the hosts. The formula to find the number of hosts is:

Number of hosts = 2ⁿ−2

Where n is the number of bits for the host part. The "-2" accounts for the network and broadcast addresses.

For 30 hosts: 2ⁿ − 2 ≥ 30

Testing values: For n = 5

2⁵−2=30 (this works)

Step 2: Determine the Number of Subnet Bits

Next, you need to determine how many bits will be used for the network part of the address. Since an IPv4 address is 32 bits in total, you subtract the host bits from 32:

Network bits=32−n=32−5=27

Step 3: Construct the Subnet Mask

Now, you convert the network bits to a subnet mask. A subnet mask uses 1s for the network part and 0s for the host part. For our example:

27 bits for the network: 11111111.11111111.11111111.11111110

This translates to decimal as: 255.255.255.254

Step 4: Verify the Subnet

Now you can verify the subnet: 

Subnet Mask: 255.255.255.254

CIDR Notation: /27

Subnetting in Cisco certification?

Subnetting is an important topic that is covered in several Cisco certification exams, including the Cisco Certified Network Associate (CCNA) and Cisco Certified Network Professional (CCNP) certifications.

The CCNA certification focuses on the basics of subnetting and how to configure Cisco routers and switches.

The CCNP certification covers advanced subnetting and routing concepts, as well as more advanced configurations for Cisco routers and switches.

To pass the CCNA, CCNP, or other Cisco certifications that include subnetting, it is important to have a good understanding of the subnetting process and be able to perform subnetting calculations.

It’s also important to have a good understanding of IP addressing schemes and IPV4/IPV6 addressing and routing. Practicing subnetting questions, doing lab exercises, and taking Practice tests will be very helpful in passing the exams.

Overall, a solid understanding of subnetting and the ability to perform subnetting calculations is essential for passing several Cisco certification exams, particularly those focused on routing and switching.