Software⏱️ 4 min read📅 2026-06-11

How to Fix: How to correctly set the subnet mask installing Linux Ubuntu 20.04 server? ERROR: '172.16.182.216' is not contained in '255.255.255.0/24'

Ubuntu Linux installation error with subnet mask

Quick Answer: Use the CIDR notation correctly by adding a slash (/) after the IP address, e.g. 172.16.182.216/24.

📋 Table of Contents

  1. ⚠️ Common Causes
  2. ✅ Best Solutions to Fix It
  3. ✨ Wrapping Up

The error '172.16.182.216' is not contained in '255.255.255.0/24' occurs when you try to set a subnet mask on an Ubuntu Server installation using static IP address, but the value provided does not match the CIDR format required by the system.

This issue can be frustrating for users who are not familiar with network configuration or do not have extensive knowledge of networking protocols. However, with the right guidance and troubleshooting steps, you can resolve this error and successfully set up your Ubuntu Server.

⚠️ Common Causes

  • The primary reason for this error is that the subnet mask value provided does not match the CIDR format required by the system. The CIDR format specifies both the subnet mask and the number of hosts in each subnet, which is represented by the slash (/) followed by the number of bits.
  • Another possible cause of this error could be a misconfigured gateway or DNS settings, but this is less likely to be the primary cause since you have provided the correct values for these settings.

✅ Best Solutions to Fix It

Using CIDR Format to Set Subnet Mask

  1. Step 1: To fix this error, you need to convert the subnet mask value from dotted decimal notation to CIDR format. To do this, calculate the number of bits required for each octet by taking the logarithm base 2 of the decimal value and rounding down to the nearest whole number.
  2. Step 2: For example, the subnet mask '255.255.255.0' would be converted to CIDR format as follows: 8-bit + 8-bit + 8-bit + 8-bit = 32 bits total, which is represented as /24 (2^24).
  3. Step 3: Once you have calculated the correct CIDR format for your subnet mask, enter this value into the Ubuntu installation prompt to set the subnet mask correctly.

Using Online Tools or Network Configuration Software

  1. Step 1: Alternatively, you can use online tools or network configuration software to convert your subnet mask to CIDR format and generate a correct IP address.
  2. Step 2: Some popular options include IPcalc, Nmap, or online tools like WhatIsMyIP.com. These tools can help you calculate the correct CIDR format for your subnet mask and provide guidance on how to configure your network settings.

✨ Wrapping Up

To summarize, the error '172.16.182.216' is not contained in '255.255.255.0/24' occurs when you try to set a subnet mask using static IP address without converting the value to CIDR format. By following the steps outlined above and using online tools or network configuration software if necessary, you can resolve this error and successfully set up your Ubuntu Server.

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