The terms netmask and subnet mask are often used interchangeably. However, subnet masks are used primarily in network configurations, while netmasks typically refer to classes of IP addresses. They are used to define a range of IP addresses that can be used by an ISP or other organization.

There are three standard classes of IP addresses – A, B, and C – which have the following netmasks:

Class A:
Class B:
Class C:

A Class A netmask defines a range of IP addresses in which the first three digit section is the same, but the other sections may each contain any number between 0 and 255. Class B addresses have the same first two sections, but the numbers in the second two sections can be different. Class C addresses have the same first three sections and only the last section can have different numbers. Therefore, a Class C IP address range may contain up to 256 addresses.

Technically, a netmask is a 32-bit value used to divide sections of IP addresses. While a class C netmask is commonly written "," it may also be defined as 11111111.11111111.11111111.00000000. This binary representation reveals the 32 bits that make up the netmask (4 sections of 8 bits each). It also shows the way the netmask "masks" the IP addresses it contains. The sections with all 1's are predefined and cannot be changed, while the section with all 0's can be any number between 0 and 255.

Updated August 10, 2009 by Per C.

quizTest Your Knowledge

The HDV video format uses what type of compression?

Correct! Incorrect!     View the HDV definition.
More Quizzes →

The Tech Terms Computer Dictionary

The definition of Netmask on this page is an original definition written by the team. If you would like to reference this page or cite this definition, please use the green citation links above.

The goal of is to explain computer terminology in a way that is easy to understand. We strive for simplicity and accuracy with every definition we publish. If you have feedback about this definition or would like to suggest a new technical term, please contact us.

Sign up for the free TechTerms Newsletter

How often would you like to receive an email?

You can unsubscribe or change your frequency setting at any time using the links available in each email.

Questions? Please contact us.