Netmask

A netmask is a 32-bit binary mask used to divide an IP address into subnets and specify the network's available hosts.

Netmask primarily provides a method to create small subnetworks from a large range of IP addresses. Generally, netmask length is defined in up to 24-bit format for all types of IP classes. The division or creation of networks into subnetworks depends on the class of IP address in use along with their available netmasks. For example, the netmasks for the three IP classes are:

• 255.0.0.0 for Class A with an 8-bit netmask
• 255.255.0.0 for Class B with a 16-bit netmask
• 255.255.255.0 for Class C with a 24-bit netmask

The greater the length of netmask the more networks it can accommodate. Therefore, the number of hosts decreases from Class A to Class C, whereas the number of available networks or subnetworks increases.

In a netmask, two of the possible addresses, represented as the final byte, are always pre-assigned and unavailable for custom assignment. For example, in 255.255.225.0, "0" is the assigned network address. In 255.255.255.255, the final "255" is the assigned broadcast address. These two values cannot be used for IP address assignment.

Below is an example of a netmask and an example of its binary conversion.

Netmask:	255.	255.	255.	255
Binary:	11111111	11111111	11111111	11111111
Netmask length	8	16	24	32

Counting out the bits in the binary conversion allows you to determine the netmask length. Above is an example of a 32-bit address. However, this address is a broadcast address and does not allow any hosts (computers or other network devices) to be connected to it.
A commonly used netmask is a 24-bit netmask, as seen below.

Netmask:	255.	255.	255.	0
Binary:	11111111	11111111	11111111	00000000
Netmask length	8	16	24	--

Using a 24-bit netmask, the network would be capable of 2,097,150 networks or 254 different hosts with an IP range of 192.0.1.x to 223.255.254.x, which is usually more than enough addresses for one network.
A simple formula can be used to determine the capable amount of networks a netmask can support.

2^(netmask length - # of used segments) - 2

For example, if we used a netmask length of 24, having a netmask of 255.255.255.0 with three used segments, subtract three from the netmask length, e.g., 24-3 = 21. With this number determined, plug it into the above formula to get 2^²¹ - 2 = 2,097,150 total number of networks. You are subtracting two from this number because of the broadcast and network addresses that are already being used.
Another example is a netmask length of 16, having a netmask of 255.255.0.0 with two used segments. Using the above formula, you would get 2^¹⁴ - 2 = 16,382 total number of networks.
To determine the number of hosts a netmask is capable of supporting, use the following formula.

2^(# of zeroes) - 2

For example, with a netmask length of 24, as shown in the above chart, there are eight zeroes. Therefore, using the formula above, this would be 2^⁸ - 2 = 254 total number of hosts. Again, two is subtracted from this number to account for the broadcast and network addresses.
Again, another example of a netmask length of 16, there would be 16 zeroes. The formula, in this case, would be 2^¹⁶ - 2 = 65,534 total number of hosts.
Below is a breakdown of each of the commonly used network classes.

Class	Netmask length	# of networks	# of hosts	Netmask
Class A	8	126	16,777,214	255.0.0.0
Class B	16	16,382	65,534	255.255.0.0
Class C	24	2,097,150	254	255.255.255.0