Subnet Mask

The subnet mask address is the most common subnet mask used on computers connected to Internet Protocol (IPv4) networks. Besides its use on home network routers, you might also encounter this mask on network professional certification exams such as the CCNA.

Subnets act as virtual fences, splitting a block of IP addresses into smaller units. This practice relieves network congestion and allows for granular access across subnets.

A subnet mask identifies individual subnets. and Subnetting

Traditional subnets worked with so-called classful networks that partitioned the IP addresses into one of five classes (Class A/B/C/D/E) according to the value of the IP address number.

The subnet mask converts to a 32-bit binary value:

  • 11111111 11111111 11111111 00000000

The 0 digits of this mask span the IP range of the subnet—8 bits or up to 256 addresses in this case. A larger number of smaller-sized subnetworks can also be defined by modifying the mask as shown in the table below.

Classful Subnets Based on 255.255.255 Mask Prefix

An incorrectly configured subnet mask (also called netmask) causes some types of network connection failures.

Subnets and CIDR

In the traditional classful scheme, many unused IP addresses were wasted because internet service providers and large corporations reserved address blocks that could not be shared.

Much of the internet subsequently converted to classless IP networking to support flexible allocation policies and cope with the surge in demand for IPv4 internet addresses in the 1990s.

Classless networks convert the traditional subnet representation to a shorthand notation based on the number of 1 digits in the mask.

 Classless Inter-Domain Routing  shorthand writes an IP address and its associated network mask in the form:


Here, n represents a number between 1 and 31 that is the number of 1 bits in the mask. 

CIDR supports classless IP addressing and associates network masks with IP network numbers independent of their traditional class. Routers that support CIDR recognize these networks as individual routes, even though they may represent an aggregation of several traditional subnets.

Network Classes

The InterNIC organization that administers internet domain names divides IP addresses into classes. The most of common of these are classes A, B, and C. Class C networks all use a default subnet mask of

Using as an IP Address

Although expressed in the form of an IP address number, network devices can only use as a mask and not as a working IP address. Attempting to use this number (or any IP number that starts with 255) as a device address causes the IP network connection to fail due to the definition of number ranges on IP networks.