255.255.255.0 Subnet Mask

The 255.255.255.0 address is the most common subnet mask used on computers connected to Internet Protocol (IPv4) networks. Besides its use on home network routers, you will also encounter this mask on network professional certification exams like the CCNA.

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

subnet mask identifies individual subnets.

255.255.255.0 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 255.255.255.0 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
MaskSubnetworksNodes/Subnet
255.255.255.01254
255.255.255.1282126
255.255.255.192462
255.255.255.224830
255.255.255.2401614
255.255.255.248326
255.255.255.252642


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 providers and large corporations reserved address blocks that could not be shared.

Much of the Internet subsequently converted to classless IP networking to support more 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:

xxx.xxx.xxx.xxx/n

where 'n' is a number between 1 and 31 that is the number of '1' bits in the mask.(For example, n=24 in the case of 255.255.255.0).

CIDR supports classless IP addressing. CIDR 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.

Using 255.255.255.0 as an IP Address

Although expressed in the form of an IP address number, network devices can only use 255.255.255.0 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 will cause the IP network connection to fail due to the definition of number ranges on IP networks.

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