What is a Netmask?

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  • Written By: Erika Peterson
  • Edited By: Bronwyn Harris
  • Last Modified Date: 10 August 2019
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A netmask is a code of numbers that is used to transfer messages correctly among machines in the same subnet. It works together with a network’s router to determine where the data goes when it is received.

Netmasks are used to identify which portion of an IP address represents the network address and which portion represents the machine address. A netmask is essentially a mathematical mask that hides a portion of an IP address and replaces it with other numbers. Netmasks are used in computer networks of all sizes.

Netmask is a term that is commonly used in the computer and internet technology industries. The most common netmask is a 32-bit mask. 32-bit masks were created to be used for two main purposes; first, to administer internet sites and divide an IP address into what are called subnets and second to specify the available hosts for the networks and determine where data goes when it is received by the router. Netmasks are essential for successful internet computer networking.

A 32-bit netmask is a component of computers that are capable of transferring data 32-bits at a time. There can also be 24-bit netmasks that are functional for smaller networks.


The netmask hides the network section of an IP address so that only the host computer or machine remains visible to network users. This is done by replacing the numbers in the IP address with a string of 1’s and 0’s. A commonly used netmask is because 255 is the decimal equivalent or sum of a string of 8 binary 1's. The 255's mask the network, and the 0 allows for the host computer part of the IP address to remain visible to anyone who accesses the network.

In a computer network the router is responsible for generating the netmask. The netmask is generated by the use of a netmask calculator. A netmask calculator is a script or a program that calculates the numbers for the netmask for the IP address. There are three classes of netmasks. They are labeled as class A, class B and class C. All classes of netmasks, A, B & C are commonly used in computer internet networks today. Different classes of netmasks are generally different lengths and are meant to be used with different sized networks and computer systems.


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