Subnet Masks

With the class-based method, the number of networks and hosts available for a particular address class is determined in advance. Therefore, an organization assigned a network identifier has a single fixed network identifier and a specified number of hosts determined by the address class to which the IP address belongs. With a single network identifier, the organization can have only a single network that connects the assigned number of hosts. If there are a large number of hosts, the network will not perform efficiently. To solve this problem, the concept of subnetting was introduced. With subnetting, a class-based network identifier can be divided into smaller (according to the defined number of identified IP addresses) network identifiers. Using these smaller network identifiers, the network can be divided into subnets, each with its own network identifier, also known as a subnet identifier.

Structure of Subnet Masks

To split a network identifier, use a subnet mask.

A subnet mask distinguishes the network identifier from the host identifier in an IP address, but is not restricted by the same rules as the class-based method. A subnet mask, like an IP address, is made up of four numbers. These numbers can range from 0 to 255.

In the class-based method, each of these four numbers can only have a maximum value of 255 or a minimum value of 0. The four numbers are then arranged as contiguous maximum values followed by contiguous minimum values. The maximum values represent the network identifier and the minimum values represent the host identifier. For example, 255.255.0.0 is a valid subnet mask as opposed to 255.0.255.0. The 255.255.0.0 subnet mask identifies the network identifier as the first two digits of the IP address.

Default Subnet Masks

With the class-based method, each address class has a standard subnet mask.

User-defined subnet masks

If subnetting is performed for the purpose of creating additional subnets for a network identifier, you can use any of the subnet masks described above with any IP address or network identifier.

The IP address 172.16.2.200 could therefore have the subnet mask 255.255.255.0.0 with the network identifier 172.16.0.0 with the network identifier 172.16.2.0 instead of the default subnet mask 255.255.0.0.

This allows an organization to split the existing Class B network identifier 172.16.0.0 into smaller network identifiers according to the actual configuration of the network.

The network identifier is the constant part of an IPv4 address.

The host identifier defines the unique and variable range within the Permitted IP Range.

Class A Net

Class B Net

Class C Net