What is a subnet mask and how to find it?

Ruth C. | December 23, 2020

A subnet mask resembles an ordinary IP address, consisting of 32 bits separated into four octets. It is also possible to represent the mask in CIDR notation in which case it is the part after the /. Another way of explaining masks is by referring to their structure. In binary, it is a string of ones followed by a bunch of zeroes. In the simplest sense, a subnet mask reveals the range IP addresses available for use. Depending on the set rules, the mask shows the exact number of devices that can connect to networks.

What is a subnet?

A subnet is a smaller network inside a larger network. Typically, people split them by using the IP address and a subnet mask. Network segmentation works as a solution to commonplace networking hurdles. For example, if criminals compromise one subnetwork, others remain safe from unauthorized access. Furthermore, separated networks are less likely to show signs of severe congestion. After the network segmentation, subnets will handle fewer hosts. Hence, this split minimizes local traffic that a single network has to manage.

However, network segmentation should prioritize the usage criteria. In other words, the generated subnets should connect hosts with similar purposes. To illustrate this rule, let’s examine a relatively simple subnetwork collection. Users divide networks according to their purpose. One subnet could serve guests exclusively. Another could accept connections from smart home gadgets. The final subnet could take care of computers and smartphones. In a way, this relates to a simple load balancing: distributing tasks to separate entities.

Understanding subnet masks

Subnet masks follow a similar structure as regular IP addresses. The latter contains two components:

  • Network ID, which indicates your network through the unique number given to it. It does not change.
  • Host ID, which represents each connected device. Note that the host part will change to refer to different devices.

In a sense, a subnet mask conceals the network part of the address, preserving only the host component. Its primary purpose is to define the exact number of IP addresses available within the network. For example, if a subnet mask indicates 256 available IP addresses, devices beyond that number will not connect.

The odds are that your network has a 255.255.255.0 subnet mask. In this case, we take a look at the first three octets (255.255.255) that cannot variate. Only “0” can be anything from 0 to roughly 256. By changing the octets, we can increase or decrease the number of available IP addresses. For example, 255.255.0.0 will supply approximately 65,536 unique IP addresses. If we take things further and use a subnet mask of 255.0.0.0, we would craft a network, facilitating over 16 million hosts.

IP addresses and subnet masks are initially binary bits, but we simplify them and write them in a dot-decimal system. We can quickly turn the subnet mask into their binary form:

  • 255.255.255.0 = 11111111.11111111.11111111.00000000
  • 255.255.0.0 = 11111111.11111111.00000000.00000000
  • 255.255.255.128 = 11111111.11111111.1111111.10000000. In this case, only 128 IP addresses are available.

Usually, a network won’t have exactly 128 or 256 usable IP addresses. The first IP address is generally for the network subnet identifier. Additionally, there is a second reserved address referred to as the network broadcast address.

How to find the subnet mask in your device?

A subnet mask will help you determine the number of devices that can connect to your network.

Windows:

  1. Open Control Panel and select Network and Sharing Center.

  1. Select your network name (next to Wi-Fi).

  1. Click on Details.

  1. Your IP address and subnet mask will be visible in this window.

macOS:

  1. Navigate to System Preferences and pick Network.
  2. Opt for the Advanced option to see more settings.

  1. Click on the TCP/IP tab, and you will see both your IP address and the applied subnet mask.

  1. An alternative is to open Terminal and type “ipconfig.” Press enter, and among the presented information, you will find the same information.

iOS:

  1. Move to Settings and find the Wi-Fi section.

  1. Look for your network and click on the “i” icon.

  1. You will see all network-related details, including the subnet mask.

Android:

  1. Navigate to Settings, and select the Wi-Fi & network section.

  1. Click on your network name in the Wi-Fi section.

  1. Choose Advanced.

  1. All the network details will be visible to you.

Final words

Understanding the subnet mask principle is highly convenient. You can quickly learn the number of IP addresses available on your network. If we consider 255.255.254.0 (11111111.11111111.11111110.00000000) as our new subnet mask, we get 510 available IP addresses. Chances are, regular netizens won't need to go beyond 255.255.255.0 default. However, environments supporting higher numbers of hosts will need to adjust their subnet masks.

Ruth C.

Ruth C.

Cybersecurity Researcher and Publisher at Atlas VPN. Interested in cybercrime, online security, and privacy-related topics.

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