Home : Internet Terms : Netmask Definition


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

Cite this definition:


TechTerms - The Tech Terms Computer Dictionary

This page contains a technical definition of Netmask. It explains in computing terminology what Netmask means and is one of many Internet terms in the TechTerms dictionary.

All definitions on the TechTerms website are written to be technically accurate but also easy to understand. If you find this Netmask definition to be helpful, you can reference it using the citation links above. If you think a term should be updated or added to the TechTerms dictionary, please email TechTerms!

Sign up for the free TechTerms Newsletter

How often would you like to receive an email?

You can unsubscribe at any time.
Questions? Please contact us.