What is Block Size in Subnetting? (Quick)

Blocks in subnetting is a set of IP addresses used by a network. These blocks usually contain a number of addresses that are a power of two. The number of addresses in a block is called the block size.

The block size is usually a power of two. This means that the block size can be formed by taking two to the power of an integer. Subnetting is basically stealing bits from the host of an IP address.

This is required to divide a network into smaller sub-networks. The result of the subnetting process is the network subnet host fields. From here, there will be two IP addresses used to identify the subnet and the broadcast subnet address.

What is Block Size in Subnetting?

To understand block size in subnetting better, imagine this. There are 256 addresses in a network. Two of these addresses are already taken as follows:

  • One to identify the network address.
  • Another one to identify the broadcast address.

Now, we split the addresses equally into 8 subnets with 32 addresses in each subnet. All of the 8 new subnets contain 2 things in common, they both have one address dedicated for the subnet and another for the broadcast address within the subnet.

Finding the block size in subnetting requires mathematic skills. To find the block size, take the subnet mask along with an IP address. The block size can be found from the range of addresses captured with a single subnet mask.

This method uses basic mathematic functions. Nowadays, there are many computer programs that can help a user find the block size. There are many of these programs both available in web-based and application-based forms.

How To Find Block Size?

In an IP address, there are many octets which must be separated when finding the block size. For example, is totally different from and must not be confused for one another in order to avoid a miscalculation.

One must understand basic subnetting which involves boundaries created by two to the power of an integer to understand this.

There are a few rules involved in finding the block size:

  • Addresses must be contiguous: one after another.
  • The number of addresses in a block must be a power of 2.
  • The first address must be divisible by the number of addresses.


To sum all of this up. Finding block size in subnetting is an essential part of subnetting and is considered basic information about a subnetwork. In the most basic form, it is the number of addresses that can be found through one single subnet mask.

There are many ways used to find the block size from calculating mathematics to using computer programs. There are also many rules to consider when finding the block size.


Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.