Netmask Troubleshooting: Common Issues and Solutions


In the world of networking, a netmask is the unsung hero, often overshadowed by IP addresses. A netmask, also known as a subnet mask, divides IP networks into subnetworks and manages network traffic. See? An unlauded hero. Because it is so important, if something goes wrong with it, connectivity could get ugly. This guide explores the ins and outs of netmask troubleshooting. So, shall we?


Firstly, What’s a Netmask and How Does it Work?

See, before we dive into the nitty-gritty details of netmask troubleshooting, understanding the concept is key (we know we gave you an overview in the introduction; we just like to be thorough). 


A netmask is a 32-bit binary mask used in IP networking to divide an IP address into network and host portions. It helps determine which part of an IP address represents the network and which part represents the device (host). For example, given an IP address of and a netmask of, the network portion is (Ones in the netmask correspond to the network portion of the IP address, while zeros represent the host portion.)


What Are Netmask Formats?

To avoid confusion, and again, to be thorough, netmasks have two formats: Dotted Decimal Notation (DDN) and CIDR Notation. The DDN format is written as four octets separated by dots, while the CIDR format is indicated by a forward slash followed by the number of bits set to 1 in the netmask. Examples of both include:


  • Dotted Decimal Notation (DDN):
  • CIDR Notation: /24


What Are Common Netmask Issues and Their Solutions?

Netmask errors happen for all sorts of reasons; it could be connectivity, IP issues, routing problems—many things, really. Nevertheless, here are the most common ones and how to fix them:


Incorrect Netmask Configuration

This is the most common culprit. When it happens, you’ll be unable to communicate with other devices on the network, and the devices will appear on the wrong subnet. So, what can you do about it? Here’s what:

  • Check the netmask configuration and make sure it matches the intended network configuration.

  • Review the netmask settings on all network devices, including routers, switches, and individual computers; they should all be consistent.

  • Cross-reference the netmask with your network plan or documentation to ensure it is correct.

  •  Use an IP calculator tool to confirm the netmask and the resulting subnet.

Overlapping Subnets

When subnets overlap, you get IP address conflicts, routing issues, and unexpected traffic behavior—not pretty. Here’s what you can do to fix this:

  • You need to identify those overlapping subnets by checking the subnet ranges in your network configuration (the overlapping subnets will have the same IP range), so you need to ensure that each subnet has a unique IP range.

  • Do a thorough review of the routing tables to identify any routes that may be causing trouble.


Incorrect CIDR Notation

When this happens, you’ll find that devices are not communicating as expected, and there are access issues. Here’s how you resolve this:

  • You make sure that the CIDR notation used in the network configuration is correct.

  • You convert the CIDR notation to dotted decimal to double-check the netmask value.


Misconfigured DHCP Server

A misconfigured DHCP server causes serious damage. These include devices receiving incorrect IP addresses and inconsistent network configurations across devices. Here’s what to do about it:

  • Carefully review the DHCP server configuration and make sure the correct netmask is assigned to devices.

  • Make sure DHCP scopes are correctly defined with the right netmask.

Routing Issues Due to Netmask

Routing issues cause traffic and network segmentation problems. Troubleshooting steps include:

  • Checking the routing tables for incorrect routes caused by misconfigured netmasks.

  • Using tools like traceroute or ping to identify where traffic is being misrouted.


VPN and Netmask Mismatch

When there’s a VPN and netmask mismatch, VPN clients won’t be able to access network resources, and there will be split tunneling issues. What can you do about it?

  •  Verify the netmask configuration for the VPN client and server.

  • Make sure split tunneling is correctly configured with the appropriate netmask.


Broadcast Storm

Finally, one of the most common netmask issues is a broadcast storm. A broadcast storm occurs when broadcast packets flood the network, overwhelming devices and causing connectivity issues; in other words, when there’s a broad, misconfigured netmask. You can fix this by:

  • Ensuring the netmask is appropriate for your network size.

  • Enabling the Spanning Tree Protocol (STP) that helps prevent loops in the network and mitigate broadcast storms.


What Tools to Use for Netmask Troubleshooting?

After you’ve learned all about the common issues and solutions of netmasks, it’s time you’ve learned what tools to use while resolving those issues. These tools include:


  • IP Calculators: These help with verifying network configurations. They determine subnet ranges, netmask values, and CIDR notation.

  • Network Analyzers: They can capture and analyze network traffic, helping identify misconfigured netmasks and routing issues. An example of a network analyzer is Wireshark.

  • Ping and Traceroute: These tools help diagnose connectivity issues and trace the path of network traffic, revealing where problems occur.


What to Do to Prevent Netmask Issues?

You know what they say: “Prevention is better than cure,” and this is quite true. Here’s what you need to do to avoid netmask-related issues:


  • Do some proper network planning before implementation, and document the network configuration to prevent overlaps and misconfigurations.

  • Conduct regular network audits to ensure configurations remain correct and consistent.

  • Use IP address management (IPAM) tools to automate and manage IP address assignments and netmask configurations.


That’s all, fellow techs! Netmasks, though seemingly simple, play a crucial role in network communication, and any issues with them can cause significant network problems. If you take care of your netmasks, they take care of your network!

HostSailor has great tools that can help with network issues. Reach out to us today!



  1. What does netmask mean?

A netmask is a tool in computer networking that splits an IP address into two parts: the network address (which identifies the network) and the host address (which identifies individual devices on the network).


  1. What are the symptoms of an incorrect subnet mask?

The symptoms of an incorrect subnet mask include:

  • Devices failing to communicate with each other on the same network.
  • Difficulty accessing the internet.


  1. How do I find my netmask?

You can check network settings on your device (often in network configuration or IP settings) or consult your network administrator.

  1. How do I fix my subnet mask?

You need to identify the issue with the subnet mask before you can fix it. It is best to consult your network administrator for proper configuration.

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