How to Determine Network Prefixes

A network prefix, often known as a subnet mask or CIDR notation, indicates which portion of an IP address is the network and which part can be used for hosts (devices). Determining network prefixes is crucial for IP addressing and routing. Here’s how to determine network prefixes:

1. From Subnet Masks:

When given a subnet mask, you can determine the prefix length by counting the number of consecutive ‘1’ bits in its binary representation.

For example, the subnet mask in binary is 11111111.11111111.11111111.00000000. So, the network prefix (or CIDR notation) is /24.

2. From CIDR Notation:

CIDR notation is written as an IP address followed by a slash (/) and the prefix length.

For example, in the address, the /24 means that the first 24 bits are the network prefix.

3. Using Network Tools:

Many networking tools can display the network prefix:

  • Command Line on Windows: Use the ipconfig command.

    This will display the IP address and subnet mask for each interface. Convert the subnet mask to CIDR notation.

  • Command Line on Linux/Mac: Use the ifconfig or ip a command.


    ip a

    Look for inet entries. It will often show the IP address and CIDR notation directly.

4. From Networking Devices:

  • If you log into your router or another networking device’s interface, you can often find the network prefix under the LAN or WAN settings, depending on what you’re looking for.

5. Online Tools and Calculators:

There are numerous online subnet calculators available. You can enter an IP address and subnet mask, and these tools will provide the CIDR notation and other useful subnetting information.


When working with IPv6, the principles are the same, but the address size is 128 bits compared to IPv4’s 32 bits. CIDR notation is still used, e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334/64.

Understanding network prefixes is crucial for network design, security, and efficient IP address management. When subnetting, the chosen prefix determines the number of subnets and hosts you can have.

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