Subnet Mask Quick Calculation Guide and Networking Error Fixes

If you've ever set up a network, a router or faced internet issues, you've likely encountered the term "subnet mask." Many people struggle when they are unsure about:

• Which one is correct to use?
• How to calculate it?
• How to fix errors that come from using the wrong subnet mask?

In this subnet blog, we will fix and configure your network. 

What are the Types of Subnet Mask?

Subnet masks are categorized as default and custom. Default masks based on traditional IP address formats and custom masks like VLSM are used for network management.

1. Default Network Mask

Standard values in IP addressing are called default subnet masks, used to divide a network into smaller segments. Each class IP addresses possess a default mask.

• Class A: Subnet mask of 255.0.0.0, which is mostly employed in very large networks.
• Class B: 255.255.0.0, which is applicable in medium-sized networks.  
• Class C: 255.255.255.0 subnet mask, designed for smaller networks.
• Class D: 255.255.255.255, used for multicast addresses rather than subnetting

1. Custom Subnet Masks (including VLSM)

Custom Subnet Masks or Variable Length Subnet Masking (VLSM), simplify networks by reducing IP address waste and increasing the number of available IP addresses. It enhances network management efficiency.

• Granular Control:  VLSM manages subnet size precisely, preventing IP address waste.
• Flexibility: Creates subnets of various sizes, tailored to network needs
• Bits Borrowing: Creates customized subnet masks using host bits.
• CIDR Notation: Uses CIDR notation for non-standard subnet masks (for example, /24, /27, /29).C

What Subnet Mask to Use?

The most common subnet mask for home networks and small businesses is subnet mask 255.255.255.0. This type of subnetting mask allows for 254 available IPs, which is usually sufficient for home or small office setups. 

1. <h3> 255.255.255.0 Subnet Mask

Subnet mask 255.255.255.0 is the most common subnet mask of a Class C network, which can be used in most of the home and small business networks. It offers a reasonable trade-off between space available in usable IP addresses and subnetting complexity.

1. 255.255.0.0 Network Mask

Class B subnets use this subnet mask and they are normally applied in medium-sized organizations. It has a significantly higher number of IP addresses in comparison to a Class C network.

1. 255.0.0.0 Subnet Mask

The Class A networks use 255.0.0.0 subnet mask which is suitable in large organizations. It supports the maximum number of IP addresses.

1. Other Masks

Advanced users can customize subnet masks using CIDR notation(e.g., /24 for 255.255.255.0, /16 for 255.255.0.0) to create smaller or larger subnets, enhancing efficiency and network management by allowing for more efficient use of IP addresses.

Factors to Consider when Choosing a Subnet Mask

The choice of a subnet mask depends on the size of your network, the division it needs, and your specific requirements. It determines the number of bits to use for identifying the network part and the host part of an address, determining the number of devices that can connect to each network segment.

Network Size

The size of your network will be a big factor in determining how many subnets you need. . 

• Smaller and Larger Networks: A smaller network might need fewer subnets compared to a larger network that may need a more accurate division.
• Total Devices: Identify how many devices (hosts) are required to be added to your network.
• Future Growth: Select a subnet mask that is sufficient to meet your immediate needs and has room to expand.

Desired Segmentation

Subnetting breaks a large network down into smaller, more manageable pieces. This improves management, performance, and security

• Improved Management: Subnetting lets you divide networks into manageable segments.
• Better Performance & Security: This can enhance security and traffic management.
• Organizational Structure: Considers organizational structure, departments, functional areas.

Specific Application Needs

Some applications or services have unique requirements, like multicast or prioritized traffic. Your subnet mask should accommodate these needs.

• Network Configurations: Specific network configurations for multicast or broadcast capabilities.
• Priority Traffic: Certain subnets may require higher priority.
• Application Requirements: Subnet masks should support application requirements.

IP Address Classes and CIDR Notation

Traditional IP classes (A, B, C) come with default subnet masks, but CIDR offers more flexibility. CIDR notation (like /24) makes subnetting clearer and easier to grasp.

• Classful IP addressing: Classful IP addressing (Classes A, B, C) has default subnet masks, but not ideal for modern networks.
• Classless Inter-Domain Routing (CIDR): It allows for flexible address allocation.
• CIDR notation: CIDR notation (e.g., /24) indicates the number of network bits in the subnet mask. For example, a /24 subnet mask (subnet 255.255.255.0) allows for 254 usable host addresses. 
• Subnet mask calculators: Subnet mask calculators simplify the process of determining the appropriate subnet mask.

How to Calculate a Subnet Mask?

To calculate network mask, you must figure out the number of network bits and host bits within an IP address.

Step 1: Find CIDR notation

This represents the number of bits used to address the part of the network. E.g., /24 indicates that the first 24 bits belong to the network.

Step 2: Convert to Binary

To represent the prefix length in binary, set the network bits to 1 and the host bits to 0, like a /24.

Step 3: Convert to Decimal

Convert each 8-bit block of binary representation into its decimal equivalent, such as 11111111 becomes 255 and 00000000 becomes 0.

Step 4: The Result is the Subnet Mask

The decimal form of the binary mask is what you use for that network. For example, a /24 subnet with the binary 11111111.11111111.11111111.00000000 will have a subnet mask 255.255.255.0.

How to Check Subnet Mask of IP?

To find the IP address subnet mask, one must know the networking setup or can use utilities such as the ipconfig command in Windows and network utilities in other operating systems. Here’s how to check subnet Mask of an IP address!

For Windows 

If you're using Windows, you can open the Command Prompt and type `ipconfig` to see the subnet mask for your active network connection.

• Open Command Prompt: Press Windows + R, type cmd and hit Enter. 
• Run ipconfig: Type ipconfig and press Enter.
• Locate Subnet Mask: Look for Subnet Mask within the network adapter section.

For MacOS

On a Mac, you can go to System Settings, click on the Network tab, select your current connection and you'll find the mask listed in the details section.

• Open System Settings: To find an IP address mask, open System Settings via Apple menu.
• Navigate to Network: Head to Network in the sidebar.
• Select Network Interface: Select active network connection from list.
• View Details: View Details by clicking Details or Advanced, selecting TCP/IP tab. The subnet mask will be displayed here.

For Linux

For Linux, open a terminal window and type either `ip a` or `ifconfig`. This will show you your IP address along with the IP mask.

• Open Terminal: Open Terminal window.
• Run ip a or ifconfig: Type ip a or ifconfig and hit Enter. Output displays network interface details.
• Identify Subnet Mask: IP address and net mask are often presented in CIDR or dotted decimal format.

For Router

To find the masking address on your router, open a web browser, log into your router's admin page using its IP address (like 192.168.1.1), and then look for the address mask setting within the network or LAN configuration.

• Open a web browser: Open web browser and enter router’s default IP address. The common ones include 192.168.1.1, 192.168.0.1, or 192.168.2.1.
• Log In: Log in using router’s administrative User Name and Password.
• Network Settings: Head to Network Settings and Lan Settings section,
• Locate the IP address configuration: Find the IP address configuration. It will display the router's IP address and  Net Mask. It is usually presented in dotted decimal format (e.g., subnet mask 255.255.255.0).

Subnet Mask Doesn’t Match Your IP Range

The subnet mask does not match your IP range error occurs when the setup of a device mismatches between its subnet mask values and its IP address. This misalignment of the IP range to subnet mask results in inappropriate communication of the network, preventing the device from detecting others on the local network or accessing the gateway.

Subnet Masks, IP Ranges, and Common Error

Subnet masks define networks, IP ranges show valid addresses and errors occur when an IP is outside the range.

Network mask

A subnet mask is a 32bit number that separates an IP address into two parts: the network and the host part. It determines the size of the network and the number of devices that need an IP address. 

IP Range

An IP range includes all valid IP addresses belonging to a specific network, determined by the subnet mask. 

The Error

An error message occurs when an IP address doesn't belong to the network defined by the subnet mask, such as when an IP address is used for a larger network but isn't part of the allowed range.

For Example

• IP: 192.168.1.50 with /25 network mask > Valid range: 192.168.1.1 – 192.168.1.126
• If your device IP is outside this range, it won’t connect.

Fix it by changing your IP or network mask to match the correct range. You can use this free IP Subnet Calculator to instantly check valid ranges.

Troubleshooting

• Check the IP address: Check the IP address you are using to make sure that it is within the proper range of the network. 
• Change the subnet mask: In case everything is right regarding the IP, then you might have needed to change the subnet mask to fit in the IP address.
• Check conflicts: No other device on the networks utilizes the same IP address. 
• Configure network settings: Configure the IP address and subnetting mask on the device or server.